Staged membrane oxidation reactor system

ABSTRACT

Ion transport membrane oxidation system comprising (a) two or more membrane oxidation stages, each stage comprising a reactant zone, an oxidant zone, one or more ion transport membranes separating the reactant zone from the oxidant zone, a reactant gas inlet region, a reactant gas outlet region, an oxidant gas inlet region, and an oxidant gas outlet region; (b) an interstage reactant gas flow path disposed between each pair of membrane oxidation stages and adapted to place the reactant gas outlet region of a first stage of the pair in flow communication with the reactant gas inlet region of a second stage of the pair; and (c) one or more reactant interstage feed gas lines, each line being in flow communication with any interstage reactant gas flow path or with the reactant zone of any membrane oxidation stage receiving interstage reactant gas.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/549,900, now U.S. Pat. No. 8,419,827, which is a divisional of U.S.patent application Ser. No. 11/758,231 titled “Staged Membrane OxidationReactor System”, filed Jun. 5, 2007, now U.S. Pat. No. 8,262,755; thespecification and claims of which are incorporated by reference and madepart of this application.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Contract NumberDE-FC26-97FT96052 between Air Products and Chemicals, Inc. and the U.S.Department of Energy. The Government has certain rights to thisinvention.

BACKGROUND

The permeation of oxygen ions through ceramic ion transport membranes isthe basis for the design and operation of high-temperature oxidationreactor systems in which permeated oxygen is reacted with oxidizablecompounds to form oxidized or partially-oxidized reaction products. Thepractical application of these oxidation reactor systems requiresmembrane assemblies having large surface areas, flow passages to contactoxidant feed gas with the oxidant sides of the membranes, flow passagesto contact reactant feed gas with the reactant sides of the membranes,and flow passages to withdraw product gas from the permeate sides of themembranes. These membrane assemblies may comprise a large number ofindividual membranes arranged and assembled into modules havingappropriate gas flow piping to introduce feed gases into the modules andwithdraw product gas from the modules.

Ion transport membranes may be fabricated in either planar or tubularconfigurations. In the planar configuration, multiple flat ceramicplates are fabricated and assembled into stacks or modules having pipingmeans to pass oxidant feed gas and reactant feed gas over the planarmembranes and to withdraw product gas from the permeate side of theplanar membranes. In tubular configurations, multiple ceramic tubes maybe arranged in bayonet or shell-and-tube configurations with appropriatetube sheet assemblies to isolate the oxidant and reactant sides of themultiple tubes.

The individual membranes used in planar or tubular module configurationstypically comprise very thin layers of active membrane materialsupported on material having large pores or channels that allow gas flowto and from the surfaces of the active membrane layers. The ceramicmembrane material and the components of the membrane modules can besubjected to significant mechanical stresses during normal steady-stateoperation and especially during unsteady-state startup, shutdown, andupset conditions. These stresses may be caused by thermal expansion andcontraction of the ceramic material and by dimensional variance causedby chemical composition or crystal structure changes due to changes inthe oxygen stoichiometry of the membrane material. These modules mayoperate with significant pressure differentials across the membrane andmembrane seals, and stresses caused by these pressure differentials mustbe taken into account in membrane module design. In addition, membranemodules have upper temperature limits above which membrane degradationand/or module damage may occur. The relative importance of thesephenomena may differ depending on the specific oxidation reactions andoperating conditions used. The potential operating problems caused bythese phenomena may have a significant negative impact on the conversionefficiency and membrane operating life of the system.

There is a need in the field of high temperature ceramic membranereactors for new membrane module and reactor system designs that addressand overcome these potential operating problems. Such designs shouldinclude features to allow long membrane life, minimum capital cost, andefficient operation over wide ranges of production rates. Embodiments ofthe invention disclosed and defined herein address these needs byproviding improved module and reactor designs for use in membraneoxidation systems.

BRIEF SUMMARY

One embodiment of the invention relates to an ion transport membraneoxidation system comprising (a) two or more membrane oxidation stages,each stage comprising a reactant zone, an oxidant zone, one or more iontransport membranes separating the reactant zone from the oxidant zone,a reactant gas inlet region, a reactant gas outlet region, an oxidantgas inlet region, and an oxidant gas outlet region; (b) an interstagereactant gas flow path disposed between each pair of membrane oxidationstages, wherein the interstage reactant gas flow path is adapted toplace the reactant gas outlet region of a first stage of the pair inflow communication with the reactant gas inlet region of a second stageof the pair such that interstage reactant gas can flow from the firststage to the second stage; and (c) one or more reactant interstage feedgas lines, each line being in flow communication with any interstagereactant gas flow path or with the reactant zone of any membraneoxidation stage receiving interstage reactant gas.

Another embodiment includes a method for generating an oxidation productgas comprising (a) providing an ion transport membrane oxidation systemhaving (1) two or more membrane oxidation stages, each stage comprisinga reactant zone, an oxidant zone, one or more ion transport membranesseparating the reactant zone from the oxidant zone, a reactant gas inletregion, a reactant gas outlet region, an oxidant gas inlet region, andan oxidant gas outlet region; (2) an interstage reactant gas flow pathdisposed between each pair of membrane oxidation stages, wherein theinterstage reactant gas flow path is adapted to place the reactant gasoutlet region of a first stage of the pair in flow communication withthe reactant gas inlet region of a second stage of the pair such thatinterstage reactant gas can flow from the first stage to the secondstage; and (3) one or more reactant interstage feed gas lines, each linebeing in flow communication with any interstage reactant gas flow pathor with the reactant zone of any membrane oxidation stage receivinginterstage reactant gas. The method includes introducing one or morereactant feed gases into the reactant gas inlet region of a first stageof the two or more membrane oxidation stages; (c) introducing an oxidantgas into any of the oxidant gas inlet regions of the two or moremembrane oxidation stages; (d) introducing a reactant interstage feedgas into any of the interstage reactant gas flow paths disposed betweenadjacent membrane oxidation stage or into any reactant zone of any stagereceiving interstage reactant gas; and (e) withdrawing an oxidation gasproduct from the reactant gas outlet region of a last stage of the twoor more membrane oxidation stages.

A related embodiment of the invention provides an ion transport membraneoxidation system comprising

(a) two or more membrane oxidation stages, each stage comprising areactant zone, an oxidant zone, one or more ion transport membranesseparating the reactant zone from the oxidant zone, a reactant gas inletregion, a reactant gas outlet region, an oxidant gas inlet region, andan oxidant gas outlet region;

(b) an interstage reactant gas flow path disposed between each pair ofmembrane oxidation stages, wherein the interstage reactant gas flow pathis adapted to place the reactant gas outlet region of a first stage ofthe pair in flow communication with the reactant gas inlet region of thesecond stage of the pair such that interstage reactant gas can flow fromthe first stage to the second stage;

(c) one or more reactant interstage feed gas lines, each line being inflow communication with any interstage reactant gas flow path or withthe reactant zone of any membrane oxidation stage receiving interstagereactant gas;

(d) one or more reactant gas feed lines in flow communication with thereactant zone of a first stage of the two or more membrane oxidationstages;

(e) a reactant gas supply manifold in flow communication with one of thereactant gas feed lines to the first stage and in flow communicationwith any of the reactant interstage feed gas lines; and

(f) a product withdrawal line adapted to withdraw an oxidation productfrom the reactant zone of the last stage of the two or more membraneoxidation stages.

Another related embodiment of the invention relates to a method forgenerating an oxidation product gas comprising (a) providing an iontransport membrane oxidation system that includes (1) two or moremembrane oxidation stages, each stage comprising a reactant zone, anoxidant zone, one or more ion transport membranes separating thereactant zone from the oxidant zone, a reactant gas inlet region, areactant gas outlet region, an oxidant gas inlet region, and an oxidantgas outlet region; (2) an interstage reactant gas flow path disposedbetween each pair of membrane oxidation stages, wherein the interstagereactant gas flow path is adapted to place the reactant gas outletregion of a first stage of the pair in flow communication with thereactant gas inlet region of a second stage of the pair such thatinterstage reactant gas can flow from the first stage to the secondstage; (3) one or more reactant interstage feed gas lines, each linebeing in flow communication with any interstage reactant gas flow pathor with the reactant zone of any membrane oxidation stage receivinginterstage reactant gas; (4) one or more reactant gas feed lines in flowcommunication with the reactant zone of a first stage of the two or moremembrane oxidation stages; (5) a reactant gas supply manifold in flowcommunication with one of the reactant gas feed lines to the first stageand in flow communication with any of the reactant interstage feed gaslines; and (6) a product withdrawal line adapted to withdraw anoxidation product from the reactant zone of the last stage of the two ormore membrane oxidation stages.

This embodiment includes the steps of (b) providing a reactant gas viathe reactant gas supply manifold, introducing reactant feed gas from themanifold into the reactant zone of the first stage, and introducingreactant gas from the manifold as reactant interstage feed gas into anyof the one or more reactant interstage feed gas lines; (c) introducingan oxidant gas into any of the oxidant gas inlet regions of the two ormore membrane oxidation stages; and (d) withdrawing an oxidation gasproduct from the reactant gas outlet region of the last stage of the twoor more membrane oxidation stages.

A further embodiment of the invention includes an ion transport membraneoxidation system comprising (a) two or more membrane oxidation stages,each stage comprising a reactant zone, an oxidant zone, one or more iontransport membranes separating the reactant zone from the oxidant zone,a reactant gas inlet region, a reactant gas outlet region, an oxidantgas inlet region, and an oxidant gas outlet region; (b) an interstagereactant gas flow path disposed between each pair of membrane oxidationstages, wherein the interstage reactant gas flow path is adapted toplace the reactant gas outlet region of a first stage of the pair inflow communication with the reactant gas inlet region of a second stageof the pair such that interstage reactant gas can flow from the firststage to the second stage; (c) one or more reactant interstage feed gaslines, each line being in flow communication with any interstagereactant gas flow path or with the reactant zone of any membraneoxidation stage receiving interstage reactant gas; (d) one or morereactant gas feed lines in flow communication with the reactant zone ofa first stage of the two or more membrane oxidation stages; (e) areactant interstage feed gas supply manifold in flow communication withany of the reactant interstage feed gas lines; and (f) a productwithdrawal line adapted to withdraw an oxidation product from thereactant zone of the last stage of the two or more membrane oxidationstages.

A further related embodiment of the invention provides an ion transportmembrane oxidation system comprising (a) two or more membrane oxidationstages, each stage comprising a reactant zone, an oxidant zone, one ormore ion transport membranes separating the reactant zone from theoxidant zone, a reactant gas inlet region, a reactant gas outlet region,an oxidant gas inlet region, and an oxidant gas outlet region; (b) aninterstage reactant gas flow path disposed between each pair of membraneoxidation stages, wherein the interstage reactant gas flow path isadapted to place the reactant gas outlet region of a first stage of thepair in flow communication with the reactant gas inlet region of asecond stage of the pair such that interstage reactant gas can flow fromthe first stage to the second stage; (c) one or more reactant interstagefeed gas lines, each line being in flow communication with anyinterstage reactant gas flow path or with the reactant zone of anymembrane oxidation stage receiving interstage reactant gas.

This embodiment includes (d) one or more reactant gas feed lines in flowcommunication with the reactant zone of a first stage of the two or moremembrane oxidation stages; (e) a reactant interstage feed gas supplymanifold in flow communication with any of the reactant interstage feedgas lines; (f) a reactant gas supply manifold in flow communication withany of (1) any of the reactant interstage feed gas lines and (2) any ofthe one or more reactant gas feed lines; and (g) a product withdrawalline adapted to withdraw an oxidation product from the reactant zone ofthe last stage of the two or more membrane oxidation stages.

An optional embodiment of the invention relates to a method forgenerating an oxidation product gas comprising (a) providing an iontransport membrane oxidation system that includes (1) two or moremembrane oxidation stages, each stage comprising a reactant zone, anoxidant zone, one or more ion transport membranes separating thereactant zone from the oxidant zone, a reactant gas inlet region, areactant gas outlet region, an oxidant gas inlet region, and an oxidantgas outlet region; (2) an interstage reactant gas flow path disposedbetween each pair of membrane oxidation stages, wherein the interstagereactant gas flow path is adapted to place the reactant gas outletregion of a first stage of the pair in flow communication with thereactant gas inlet region of a second stage of the pair such thatinterstage reactant gas can flow from the first stage to the secondstage; (3) one or more reactant interstage feed gas lines, each linebeing in flow communication with any interstage reactant gas flow pathor with the reactant zone of any membrane oxidation stage receivinginterstage reactant gas; (4) one or more reactant gas feed lines in flowcommunication with the reactant zone of a first stage of the two or moremembrane oxidation stages; (5) a reactant interstage feed gas supplymanifold in flow communication with any of the reactant interstage feedgas lines; and (6) a product withdrawal line adapted to withdraw anoxidation product from the reactant zone of the last stage of the two ormore membrane oxidation stages.

This embodiment includes the steps of (b) introducing a reactant feedgas into the reactant zone of the first stage of the two or moremembrane oxidation stages; (c) providing reactant interstage feed gasvia the reactant interstage feed gas supply manifold into any of the oneor more reactant interstage feed gas lines; (d) introducing an oxidantgas into any of the oxidant gas inlet regions of the two or moremembrane oxidation stages; and (e) withdrawing an oxidation gas productfrom the reactant gas outlet region of the last stage of the two or moremembrane oxidation stages.

Another embodiment provides a method for the operation of an iontransport membrane oxidation system comprising (a) introducing one ormore reactant gases into a reactant zone of the ion transport membraneoxidation system, wherein the one or more reactant gases comprise atleast methane and carbon dioxide; (b) introducing an oxygen-containinggas into an oxidant zone of the ion transport membrane oxidation system;(c) permeating oxygen from the oxidant zone through an ion transportmembrane into the reactant zone and reacting the oxygen therein with oneor more components in the reactant gases; and (d) maintaining thepartial pressure of carbon dioxide in the reactant gas flowing into thereactant zone to be less than a critical threshold carbon dioxidepartial pressure, p_(CO2)*, wherein p_(CO2)* is defined as a carbondioxide partial pressure above which the material in the ion transportmembrane reacts with carbon dioxide and decomposes.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic flow diagram of a generic embodiment of thepresent invention.

FIG. 2 is a schematic flow diagram of an embodiment of the invention.

FIG. 3 is a schematic flow diagram for a specific configuration of theembodiment of FIG. 2.

FIG. 4 is a schematic flow diagram of another embodiment of theinvention.

FIG. 5 is a plot of P_(CO2)* vs. temperature at various equilibrium O₂partial pressures for the mixed conducting metal oxide membrane materialLa_(0.9)Ca_(0.1)FeO_(3-δ).

FIG. 6 is a plot of the temperature of the reactant gas as a percentageof a group of 100 reaction stages from the reactant feed inlet (0) tothe product outlet (100) for Example 1.

FIG. 7 is a plot of the temperature of the reactant gas as a percentageof a group of 100 reaction stages from the reactant feed inlet (0) tothe product outlet (100) for Example 2.

FIG. 8 is a plot of the temperature of the reactant gas as a percentageof a group of 100 reaction stages from the reactant feed inlet (0) tothe product outlet (100) for Example 3.

DETAILED DESCRIPTION

Membrane oxidation reactor systems typically utilize partial oxidation,complete oxidation, steam reforming, carbon dioxide reforming, water-gasshift, and/or various combinations of these reactions to producesynthesis gas. Certain of these reactions are strongly exothermic andothers are endothermic. Because ceramic membrane systems generallyrequire a narrow operating temperature range, proper control of theexothermic and endothermic reactions is required. This need is addressedby embodiments of the present invention directed toward the design andoperation of ion transport membrane (ITM) systems that utilize multiplereactant-staged membrane modules operating in series for use inoxidation processes. It has been found that when exothermic reactionsoccur between permeated oxygen and reactive components, for example inthe production of synthesis gas from methane, the degree of reactantconversion across an individual membrane must be limited to prevent anexcessive temperature gradient across the membrane. It also has beenfound that when a membrane is transporting oxygen, the amount of oxygenextraction across an individual membrane must be limited to prevent anexcessive oxygen vacancy gradient in the membrane material between theleading edge and trailing edge of the membrane. Excessive temperature oroxygen vacancy gradients may cause excessive stresses in the membranesthat could seriously limit the membrane life. In addition, it has beenfound that the concentration of certain components in the reactant gas,particularly carbon dioxide, must be maintained below certain criticalpartial pressures to avoid damage to the membrane material.

The embodiments of the present invention address these problems byutilizing multiple reactor stages so that the reactant conversion ineach stage can be controlled, the amount of oxygen extracted acrossmembranes in each module can be kept sufficiently low to prevent anexcessive oxygen vacancy gradient in the membrane material, and thepartial pressure of carbon dioxide in contact with the membranes can bemaintained below a critical partial pressure. A reactor stage maycomprise multiple membrane modules arranged in parallel and/or series.The amount of oxygen extracted across each individual module may belimited by appropriate module sizing, and the total desired degree ofoxygen extraction within a stage may be achieved by operating a selectedplurality of modules within a stage.

The total desired conversion in the system may be achieved by utilizinga plurality of reactor stages in series wherein each stage is operatedso that the degree of reactant conversion in each stage is controlled ata selected value and may approach chemical equilibrium. This may beaccomplished by introducing portions of the reactant gas into two ormore stages of a multiple-stage reactor system wherein each stage maycomprise multiple membrane modules arranged in series and/or parallel.The degree of reactant conversion across each individual module in astage may be controlled by appropriate module sizing and/or feed gasflow rate. Recycle gas from downstream synthesis gas conversionprocesses or reactant gas from other sources may be introduced into themembrane reactor system as additional reactant gas to increase overallconversion and/or to control temperatures in selected stages. Thisrecycle gas often contains carbon dioxide, and the partial pressure ofthis carbon dioxide at any point in the reactor may be controlled toprevent membrane degradation as described in detail below.

The following definitions apply to terms used in the description andclaims for the embodiments of the invention presented herein.

An ion transport membrane module is an assembly of a plurality of iontransport membrane structures that has a gas inflow region and a gasoutflow region disposed such that gas can flow across the externalsurfaces of the membrane structures. The plurality of ion transportmembrane structures also may have a gas inflow region and a gas outflowregion disposed such that gas can flow across the internal surfaces ofthe membrane structures. Gas flowing from the inflow region to theoutflow region of a membrane module may change in composition as itpasses across the surfaces of the membrane structures in the module.Each membrane structure has an oxidant gas feed side or zone, alsodescribed as the oxidant or cathode side, and a reactant, permeate, oranode side or zone separated by an active membrane layer or region thatallows oxygen ions to permeate through the membrane and react withreactive components on the reactant side. In one exemplary type ofmembrane module design, each module has an interior region and anexterior region wherein the oxidant gas feed flows through the interiorregion and the reactant gas flows through the exterior region of themembrane structure.

An ion transport membrane comprises an active layer of ceramic membranematerial comprising mixed metal oxides capable of transporting orpermeating oxygen ions at elevated temperatures. The ion transportmembrane also may transport electrons in addition to oxygen ions, andthis type of ion transport membrane typically is described as a mixedconductor membrane. The ion transport membrane also may be a compositemembrane comprising a layer of dense active membrane material and one ormore layers of porous or channeled support layers.

The terms “stage”, “reaction stage”, and “reactor stage” in amulti-stage membrane oxidation system are equivalent and are defined asan assembly of one or more membrane modules arranged in parallel and/orseries within the stage wherein each stage comprises (1) a reactant sideor zone (these terms are equivalent), (2) an oxidant side or zone, (3)one or more ion transport membranes separating the oxidant zone from thereactant zone, (4) a reactant gas inlet or inlet region in flowcommunication with the reactant zone, and (5) a reactant gas outlet oroutlet region in flow communication with the reactant zone. Morespecifically, each stage may have a reactant feed gas inlet or inletregion (if it is the first stage), an interstage reactant gas streaminlet or inlet region (if it is not the first stage), an interstagereactant gas stream outlet or outlet region (if it is not the laststage), and a product gas outlet or outlet region (if it is the laststage). Each stage also has one or more oxidant gas inlets or inletregions in flow communication with the oxidant zone and one or moreoxygen-depleted oxidant gas outlets or outlet regions in flowcommunication with the oxidant zone.

The one or more ion transport membranes separating the oxidant zone fromthe reactant zone allow the permeation of oxygen ions through themembrane, and the dense active membrane material does not allow anysubstantial bulk flow of gas between the oxidant and reactant zones ofthe stage. Small but acceptable leaks in the membrane may occur in somecases.

A stage may have any number of individual membrane modules arranged inseries and/or parallel flow arrangement with respect to the reactant gasstreams. Reactant gas is introduced into the stage inlet, is distributedamong the modules in the stage, and passes through the reactant sides ofthe modules. The effluent gas from the modules is withdrawn via thestage outlet. A stage may include one or more catalysts to enhance thereactions occurring therein; catalysts may include any of oxidationcatalyst, steam reforming catalyst, carbon dioxide reforming catalyst,and water gas shift catalyst. Catalyst may be disposed (1) following thereactant zone of any stage and/or (2) downstream of any membrane modulewithin any stage and/or (3) upstream of any membrane module within anystage and/or (4) within or between the modules within any stage in anydesired configuration.

A reactant gas is defined as a gas comprising one or more reactivecomponents that participate in any of the reactions (1) that occur inthe reactant zone of a membrane oxidation reactor stage and (2) that mayoccur in a catalyst region following or preceding the reactant zone ofthe membrane oxidation reactor stage. The reactions in the reactant zonemay occur (1) between oxygen permeated through the membrane and any ofthe reactive components and (2) among any of the reactive components.These reactions form reaction products that may be withdrawn as outletor product gas from any stage of a staged reactor system.

The term “hydrocarbon” is defined as a compound comprising at leasthydrogen and carbon atoms. The term “oxygenated hydrocarbon” is definedas a compound comprising at least hydrogen, carbon, and oxygen atoms.

The term “pre-reformed natural gas” means the reaction products from thecatalytic reforming of a portion of the hydrocarbons in a natural gasstream. Pre-reformed natural gas typically comprises methane, carbonmonoxide, carbon dioxide, hydrogen, and water. Pre-reforming of naturalgas may be carried out to convert and decompose hydrocarbons heavierthan methane for the purpose of eliminating coking in downstreamreforming or partial oxidation processes. The terms “pre-reforming” and“pre-reformed” mean the partial reforming of a hydrocarbon-containingstream prior to further reaction in a membrane reactor system. The terms“pre-reforming” or “pre-reformed” also may be defined as the catalyticreaction of water and/or carbon dioxide with a portion of thehydrocarbons in a hydrocarbon-containing stream, particularlyhydrocarbons heavier than methane, to form reformed products.

In one embodiment of the invention, a staged reactor system is operatedto generate a synthesis gas product comprising hydrogen and carbonmonoxide. This embodiment utilizes a first reactant gas comprising oneor more hydrocarbons and a second reactant gas comprising steam(vaporized water). A typical first reactant gas is natural gascomprising mostly methane and smaller concentrations of hydrocarbonshaving 2 to 6 carbon atoms; another typical first reactant gas ismethane-rich gas resulting from pre-reforming natural gas with steam;other carbonaceous reactant gases may be used in alternativeapplications. Optionally, a third reactant gas may be used thatcomprises one or more components selected from the group consisting ofoxygen, nitrogen, hydrogen, water, methane, other hydrocarbons, carbonmonoxide, and carbon dioxide. The third reactant gas may be provided,for example, by offgas from a downstream process that uses the synthesisgas product as feed gas. When three reactant gases are used in thisembodiment, the chief reactive components are any of methane, otherhydrocarbons, water, hydrogen, carbon monoxide, and carbon dioxide.

An oxidant gas is defined as a gas comprising oxygen and othercomponents from which oxygen can be extracted by permeation through anion transport membrane to react with reactive components in the reactantzone. An oxygen-containing gas is a type of reactant gas that comprisesone or more compounds that contain oxygen atoms such as, for example,water, carbon monoxide, and carbon dioxide.

A reactant feed gas is defined as a reactant gas introduced into thereactant zone or side of the first stage of a multi-stage membranereactor system. An interstage reactant gas is defined as reactant gasflowing between stages, i.e., from the outlet region of the reactantzone of one stage and into the inlet region of the reactant zone of thenext stage; this gas comprises reaction products and may includeunreacted reactive components. The relative amounts of reactivecomponents and oxidation products in an interstage reactant gas streamentering a given stage may depend on (1) the degree to which chemicalequilibrium is approached in the previous stage and (2) the amount andcomposition of reactant interstage feed gas (if any) that is introducedinto the interstage reactant gas to the reaction zone of the givenstage.

A reactant interstage feed gas is defined as a reactant gas that is (1)introduced into the reactant zone of any stage other than the firststage or (2) mixed with an interstage reactant gas prior to enteringthat reactant zone. The reactant interstage feed gas may have the samecomposition as the reactant feed gas to the first stage or may have adifferent composition than the reactant feed gas. A reactant feed gas orreactant interstage feed gas typically comprises high concentrations ofreactive components. A product gas is the reactant gas effluent from thereactant zone of the last stage of a multi-stage membrane reactorsystem, wherein the product gas comprises one or more oxidation productsand also may comprise unreacted reactive components. The product gas maybe, for example, synthesis gas containing at least hydrogen and carbonoxides.

A reactant-staged membrane reactor system is defined as a systemcomprising two or more membrane stages arranged in series with respectto the flow of reactant gases through the system wherein the reactantgas effluent flows from the outlet region of one stage into the inletregion of another stage immediately downsteam. Reactant feed gas entersthe first stage, product gas is withdrawn from the last stage, and aninterstage reactant gas stream flows between each pair of series stages.A reactant interstage feed gas comprising additional reactant gas may beintroduced into at least one interstage reactant gas stream and may beintroduced into any of the interstage reactant gas streams in themulti-stage reactor system.

A membrane module may have a configuration of multiple planar wafers inwhich each wafer has a center or interior region and an exterior region,wherein the wafer is formed by two parallel planar members sealed aboutat least a portion of the peripheral edges thereof. Oxygen ions permeatethrough active membrane material that may be placed on either or bothsurfaces of a planar wafer. Gas can flow through the center or interiorregion of the wafer, and the wafer has one or more gas flow openings toallow gas to enter and/or exit the interior region of the wafer. Thusoxygen ions may permeate from the exterior region into the interiorregion, or conversely may permeate from the interior region to theexterior region. In one embodiment, the gas in contact with the outersurfaces in the exterior regions of the membrane modules may be at ahigher pressure than the gas within the interior regions of the membranemodules. Representative membrane compositions and planar membrane moduleconfigurations that may be used in the embodiments of the presentinvention are described in U.S. Pat. No. 7,179,323 and U.S. PatentPublication 2004/0186018(A1), which are incorporated herein byreference.

Alternatively, the membrane module may have a tubular configuration inwhich an oxidant gas flows in contact with one side of the tube (i.e.,in either the interior region or the exterior region of the tube) andoxygen ions permeate through active membrane material in or on the tubewalls to the other side of the tube. The oxidant gas may flow inside oroutside of the tube in a direction generally parallel to the tube axis,or conversely may flow over the outer side of the tube in a directionwhich is not parallel to the tube axis. A module may comprise multipletubes arranged in bayonet or shell-and-tube configurations withappropriate tube sheet assemblies to isolate the oxidant and reactantsides of the multiple tubes.

Modules may be arranged in series within a reactor stage wherein anumber of modules are disposed along a single axis. Typically, reactantgas which has passed across the surfaces of the membrane structures in afirst module flows from the outflow region of that module, after whichsome or all of this gas enters the inflow region of a second module andthereafter flows across the surfaces of the membrane structures in thesecond module. The axis of a series of single modules may be parallel ornearly parallel to the overall flow direction or axis of the gas passingover the modules in series.

Modules may be arranged within a stage in banks of two or more parallelmodules wherein a bank of parallel modules lies on an axis that is notparallel to, and may be generally orthogonal to, the overall flowdirection or axis of the gas passing over the modules. Multiple banks ofmodules may be arranged in series, which means by definition that banksof modules are disposed such that at least a portion of reactant gaswhich has passed across the surfaces of the membrane structures in afirst bank of modules flows across the surfaces of the membranestructures in a second bank of modules.

Any number of single modules or banks of modules may be arranged inseries and/or parallel within a stage. In one embodiment, the modules ina series of single modules or in a series of banks of modules may lie ona common axis or common axes in which the number of axes equals one orequals the number of modules in each bank. In another embodiment,successive modules or banks of modules in a series of modules or banksof modules may be offset in an alternating fashion such that the moduleslie on at least two axes or on a number of axes greater than the numberof modules in a bank, respectively. Both of these embodiments areincluded in the definition of modules in series as used herein.

The term “in flow communication with” as applied to a first and secondregion means that fluid can flow from the first region to the secondregion, through an intermediate region. The intermediate region maycomprise connecting piping between the first and second regions or maycomprise an open flow area or channel between the first and secondregions. The term “connected to” as applied to a first and second regionmeans that fluid can flow from the first region directly to the secondregion or through connecting piping to the second region. The term“direct flow communication” and the term “directly” as applied to aflowing fluid mean that the fluid can flow from a first region to asecond region, and/or from the second region to the first region,wherein the flow path between the regions is not in flow communicationwith any vessel, storage tank, or process equipment, except that thefluid flow path may include piping and/or one or more flow controldevices selected from orifices, valves, and other flow restrictiondevices.

The indefinite articles “a” and “an” as used herein mean one or morewhen applied to any feature in embodiments of the present inventiondescribed in the specification and claims. The use of “a” and “an” doesnot limit the meaning to a single feature unless such a limit isspecifically stated. The definite article “the” preceding singular orplural nouns or noun phrases denotes a particular specified feature orparticular specified features and may have a singular or pluralconnotation depending upon the context in which it is used. Theadjective “any” means one, some, or all indiscriminately of whateverquantity. The term “and/or” placed between a first entity and a secondentity means one of (1) the first entity, (2) the second entity, and (3)the first entity and the second entity.

A schematic flow diagram of a generic embodiment of the invention isillustrated in FIG. 1. The exemplary membrane oxidation system comprisesfirst stage 1, second stage 3, third stage 5, and last or n^(th) stage7. Any desired number of stages may be used as long as there are atleast two stages. Each stage is illustrated schematically as a genericmodule having an oxygen permeable membrane that divides the module intoan oxidant side and a permeate or reactant side. As explained above, astage can comprise any number of membrane modules arranged in seriesand/or parallel and may include one or more catalysts.

First stage 1 comprises oxidant side or zone 1 a, membrane 1 b, reactantside or zone 1 c, optional catalyst 1 d, and appropriate gas inlet andoutlet regions. Optional catalyst 1 d is shown here as immediatelyfollowing the module. Alternatively or additionally catalyst may bedisposed immediately preceding the module (not shown) or within oraround the module in any desired configuration (not shown). Similarly,second stage 3 comprises oxidant side 3 a, membrane 3 b, reactant side 3c, appropriate gas inlet and outlet regions, and optional catalyst 3 d,which is shown here as immediately following the module. Alternativelyor additionally, catalyst may be disposed immediately preceding themodule (not shown) or within or around the module in any desiredconfiguration (not shown). Similarly, third stage 5 comprises oxidantside 5 a, membrane 5 b, reactant side 5 c, appropriate gas inlet andoutlet regions, and optional catalyst 5 d. Optional catalyst 5 d isshown here as immediately following the module. Alternatively oradditionally catalyst may be disposed immediately preceding the module(not shown) or within or around the module in any desired configuration(not shown). Last or n^(th) stage 7 comprises oxidant side 7 a, membrane7 b, reactant side 7 c, appropriate gas inlet and outlet regions, andoptional catalyst 7 d, shown here as immediately following the module.Alternatively or additionally, catalyst may be disposed immediatelypreceding the module (not shown) or within or around the module in anydesired configuration (not shown). Product gas from last stage 7 iswithdrawn via product line 7 e.

In the illustration of FIG. 1, interstage reactant gas flows from stage1 via flow path 1 e, from stage 3 via flow path 3 e, and from stage 5via flow path 5 e. In one embodiment, each of stages 1, 3, and 5 may beenclosed in a separate pressure vessel 2, 4, 6, 8; in this case, flowpaths 1 e, 3 e, and 5 e are pipes, conduits, or closed channels betweenthe vessels. In another embodiment, stages 1, 3, 5, and 7 may beenclosed in a single pressure vessel (not shown) such that reactant gascan flow through the reactant zones of each stage in succession; in thiscase, flow paths 1 e, 3 e, and 5 e are open regions between stagesthrough which gas can flow from the reactant gas outlet region of onestage into the reactant gas inlet region of the following stage. Eachstage is adjacent a downstream stage and/or an upstream stage; the firststage is adjacent a downstream stage, the last stage is adjacent anupstream stage, and all other stages are adjacent an upstream stage anda downstream stage. The terms “upstream” and “downstream” are definedrelative to the flow direction of reactant gas.

The oxidant zone and the reactant zone in each stage are isolated fromeach other so that the bulk flow of oxidant gas through the oxidant zoneand the bulk flow of reactant gas through the reactant zone are separateand independent. The membrane or membranes separating the oxidant zonefrom the reactant zone prevents any substantial bulk gas flow betweenthe zones and allows the permeation of oxygen through the membrane fromthe oxidant zone to the reactant zone. In some cases, small butacceptable leaks may occur through imperfections in the membrane.

An oxidant gas, for example, preheated air or oxygen-containingcombustion products from a combustor operated with excess air, isintroduced via oxidant inlet line 9 into oxidant side 1 a of first stage1 and contacts the oxidant side of membrane 1 b, a portion of the oxygenpermeates through membrane 1 b, and oxygen-depleted gas exits firststage 1 via oxygen-depleted oxidant outlet line 11. Similarly,additional oxidant gas streams may be introduced via lines 13, 15, and19 into stages 3, 5, and 7, respectively, and oxygen-depleted gas mayexit the stages via lines 21, 23, and 25, respectively. Alternatively,some or all of the oxidant gas may flow through two or more stages inseries via lines 57, 59, and 61. In one embodiment, for example, oxidantgas may flow through lines 9, 57, and 21 such that stages 1 and 3operate in series with respect to oxidant gas; similarly, a singleoxidant stream may provide oxidant to a pair of downstream stages. Thusthe stages may be operated individually with respect to the flow ofoxidant gas, may be operated in series with respect to the flow ofoxidant gas, or may utilize any combination of individual and seriesoperation with respect to the flow of oxidant gas. Oxidant gas inlet andoutlet manifolds (not shown) may be used to introduce oxidant gas intothe oxidant zones of the multiple stages and withdraw oxygen-depletedoxidant gas from the oxidant zones of the multiple stages.

Other oxidant gas flow configurations are possible as alternatives tothat described above. For example, the oxidant gas may flowcounter-current to the reactive gas flow, cross-flow to the reactive gasflow, or in any other arrangement such that sufficient oxidant gas isprovided to the oxidant zone side of the membranes.

Reactant gas may enter the multi-stage reactor system via manifold 27, afirst portion may be withdrawn via line 29 and combined with anotherreactant gas (for example, steam) provided in line 31, and the combinedgas may be introduced via reactant gas inlet line 33 into reactant side1 c of first stage 1. Additional portions of reactant gas may bewithdrawn from manifold 27 via any of reactant interstage feed gas lines35, 37, and 39 and introduced as reactant interstage feed gas into anyof interstage reactant gas flow paths 1 e, 3 e, and 5 e, respectively.Alternatively, the reactant interstage feed gas may be introduceddirectly into the reactant side of any stage and/or upstream of any ofthe catalysts 1 d, 3 d, 5 d, and 7 d. The reactant gas in manifold 27may comprise one or more hydrocarbons and also may comprise any of thecomponents water, carbon monoxide, carbon dioxide, and hydrogen. Forexample, the reactant gas in manifold 27 may be pre-reformed natural gascomprising methane, carbon monoxide, carbon dioxide, hydrogen, andwater. The reactant gas provided via line 31 may be, for example,vaporized water (steam).

Additional reactant gas may be provided via manifold 41 from a sourcedifferent than the source of the reactant gas in line 27 and the sourceof reactant gas provided via line 31. This additional reactant gas maybe introduced via any of lines 29, 43, 45, and 47 into any of firststage 1, the interstage reactant gas in line 1 e, the interstagereactant gas in line 3 e, and the interstage gas entering last or n^(th)stage 7. Alternatively, the additional reactant gas may be introducedupstream of any of catalysts 1 d, 3 d, 5 d, and 7 d. This additionalreactant gas may be, for example, an oxygen-containing gas comprisingcarbon dioxide obtained from a downstream process that uses product gasfrom line 7 e. The additional reactant gas may comprise unreacted offgasfrom a downstream process that uses product gas from line 7 e and/or maycomprise partially-reformed unreacted offgas from a downstream processthat uses product gas from line 7 e. Any number of additional stages maybe utilized between stage 5 and last stage 7 as desired.

The gas flow rates in any of the lines described above may be regulatedby control valves or other flow devices (not shown) known in the art.Alternatively or additionally, the temperatures of any of the gasstreams may be controlled by heating and/or cooling (not shown) bymethods known in the art.

Various combinations of reactant gas types including oxygen-containinggas may be introduced into the reactant sides of the modules in thestaged membrane oxidation reactor system of FIG. 1. In one embodiment,for example, pre-reformed natural gas may be introduced into the reactorstages via manifold 27 and lines 29, 35, 37, and 39, and steam may beintroduced via lines 31 and 33 into first stage 1. No additionalreactant gas is provided via manifold 41 and lines 29, 43, 45, and 47 inthis embodiment. In another exemplary embodiment, pre-reformed naturalgas and steam may be introduced into the first stage via lines 31 and33, and carbon dioxide-containing gas (for example, a recycle gas from adownstream process) may be introduced into the system via manifold 41and any of lines 43, 45, and 47. The downstream process may be ahydrocarbon synthesis process (e.g., a Fischer-Tropsch process) or anoxygenated hydrocarbon synthesis process (e.g., an alcohol synthesisprocess). The downstream process may utilize synthesis gas produced bythe staged oxidation reactor system of FIG. 1. Manifold 27 and lines 29,35, 37, and 39 are not used in this embodiment. Other embodiments arepossible in which combinations of reactant gas from different sourcesare introduced into the reactor stages. For example, pre-reformednatural gas may be provided to the staged reactor system via manifold 27and lines 29, 35, 37, and 39, steam may be introduced into the systemvia line 31, and carbon dioxide-containing gas (for example, a recyclegas from a downstream process) may be introduced into the system viamanifold 41 and any of lines 43, 45, and 47. In another example, areactant gas comprising pre-reformed natural gas, a carbondioxide-containing additional reactant gas, and steam are provided tothe staged reactor system via manifold 27 and lines 29, 35, 37, and 39,and steam may be introduced into the system via line 31. Manifold 41 andlines 43, 45, and 47 are not used in this case.

Another embodiment of the invention is illustrated in the schematicflowsheet of FIG. 2 wherein a membrane oxidation system comprises firststage 201, second stage 203, third stage 205, and last or n^(th) stage207. Any number of stages may be used as long as there are at least twostages. Each stage is illustrated schematically as a generic modulehaving an oxygen permeable membrane that divides the module into anoxidant side or zone and a permeate or reactant side or zone. Asexplained above, a stage can comprise any number of membrane modulesarranged in series and/or parallel and may include one or more catalystsselected from oxidation catalyst, steam reforming catalyst, carbondioxide reforming catalyst, and water gas shift catalyst.

First stage 201 comprises oxidant side 201 a, membrane 201 b, reactantside 201 c, optional catalyst 201 d, and appropriate gas inlet andoutlet regions. Optional catalyst 201 d is shown here as immediatelyfollowing the module, but alternatively or additionally catalyst may bedisposed immediately preceding the module (not shown) or within oraround the module in any desired configuration (not shown). Similarly,second stage 203 comprises oxidant side 203 a, membrane 203 b, reactantside 203 c, appropriate gas inlet and outlet regions, and optionalcatalyst 203 d, which is shown here as immediately following the module.Alternatively or additionally, catalyst may be disposed immediatelypreceding the module (not shown) or within or around the module in anydesired configuration (not shown). Similarly, third stage 205 comprisesoxidant side 205 a, membrane 205 b, reactant side 205 c, appropriate gasinlet and outlet regions, and optional catalyst 205 d. Optional catalyst205 d is shown here as immediately following the module, butalternatively or additionally catalyst may be disposed immediatelypreceding the module (not shown) or within or around the module in anydesired configuration (not shown). Last or n^(th) stage 207 comprisesoxidant side 207 a, membrane 207 b, reactant side 207 c, appropriate gasinlet and outlet regions, and optional catalyst 207 d, shown here asimmediately following the module. Alternatively or additionally,catalyst may be disposed immediately preceding the module (not shown) orwithin or around the module in any desired configuration (not shown).Product gas from last stage 207 is withdrawn via product line 207 e.Interstage reactant gas flows from stage 201 via line 201 e, from stage203 via line 203 e, and from stage 205 via line 205 e.

An oxidant gas, for example, preheated air or oxygen-containingcombustion products from a combustor operated with excess air, isintroduced via line 209 into oxidant side 201 a of first stage 201 andcontacts the oxidant side of membrane 201 b, a portion of the oxygenpermeates through membrane 201 b, and oxygen-depleted gas exits firststage 201 via line 211. Similarly, additional oxidant gas streams may beintroduced via lines 213, 215, and 219 into stages 203, 205, and 207,respectively, and oxygen-depleted gas may exit the stages via lines 221,223, and 225, respectively.

Natural gas is provided as a reactant gas via line 227, is mixed withsteam from line 229, the mixture is heated in pre-heater 231, and theheated mixture flows via line 233 to steam-methane reformer 235.Hydrogen may be added via line 237 for use within the reformer for feedgas desulfurization (not shown) as is normally practiced in thesteam-methane reforming art. Partially-reformed or pre-reformed gasexits the reformer via line 239 and optionally is mixed with reactantgas in line 241 (for example, a recycle gas from a downstream process)to form a reactant feed gas flowing through manifold 243. The downstreamprocess may be a hydrocarbon synthesis process (e.g., a Fischer-Tropschprocess) or an oxygenated hydrocarbon synthesis process (e.g., analcohol synthesis process). The downstream process may utilize synthesisgas produced by the staged oxidation reactor system of FIG. 2. Reactantfeed gas may be at a temperature of 600 to 1150° C. and a pressure of 2to 40 atma, and the gas typically comprises methane, water, hydrogen,carbon dioxide, and carbon monoxide.

A first portion of the reactant gas feed via line 245 is mixed withsteam provided in line 247 and the mixed reactant feed gas flows intoreactant side 201 c of first stage 201, reacts therein with oxygenpermeated through membrane 201 b, passes through optional catalyst 201d, and flows via flow path 201 e as an interstage reactant gas. A secondportion of the reactant gas feed from manifold 243 is withdrawn via line249 to provide a reactant interstage feed gas that is mixed with theinterstage reactant gas in flow path 201 e. The mixed gas then flowsinto reactant side 203 c of second stage 203, reacts therein with oxygenpermeated through membrane 203 b, passes through optional catalyst 203d, and flows via flow path 203 e as an interstage reactant gas.Alternatively, the reactant interstage feed gas in line 249 may beintroduced directly into the reactant side or zone of stage 203 and/orupstream of catalyst 201 d.

A third portion of the reactant gas feed from manifold 243 is withdrawnvia line 251 to provide a reactant interstage feed gas that is mixedwith the interstage reactant gas in flow path 203 e. The mixed gas thenflows into reactant side 205 c of third stage 205, reacts therein withoxygen permeated through membrane 205 b, passes through optionalcatalyst 205 d, and flows via flow path 205 e as an interstage reactantgas. Alternatively, the reactant interstage feed gas in line 251 may beintroduced directly into the reactant side of stage 205 and/or upstreamof catalyst 203 d.

A fourth or n^(th) portion of the reactant gas feed from manifold 243 iswithdrawn via line 253 to provide a reactant interstage feed gas that ismixed with the interstage reactant gas in flow path 205 e. The mixed gasthen flows into reactant side 207 c of last or n^(th) stage 207, reactstherein with oxygen permeated through membrane 207 b, passes throughoptional catalyst 207 d, and flows via line 207 e as a product synthesisgas. Alternatively, the reactant interstage feed gas in line 253 may beintroduced directly into the reactant side of stage 207 and/or upstreamof catalyst 205 d.

The product gas may be at a temperature of 600 to 1150° C. and apressure of 2 to 40 atma, and the gas may typically comprise hydrogen,carbon monoxide, water, carbon dioxide, and methane. Any number ofadditional stages may be utilized between third stage 205 and last stage207 as desired.

In the illustration of FIG. 2 described above, interstage reactant gasflows from stage 201 via flow path 201 e, from stage 203 via flow path203 e, and from stage 205 via flow path 205 e. In one embodiment, eachof stages 201, 203, 205, and 207 may be enclosed in a separate pressurevessel 202, 204, 206, and 208; in this case, flow paths 201 e, 203 e,and 205 e are pipes, conduits, or closed channels between the vessels.In another embodiment, stages 201, 203, 205, and 207 may be enclosed ina single pressure vessel (not shown) such that reactant gas can flowthrough the reactant zones of each stage in succession; in this case,flow paths 201 e, 203 e, and 205 e are open regions between stagesthrough which gas can flow from the reactant gas outlet region of onestage into the reactant gas inlet region of the following stage.

A selected embodiment of the system of FIG. 2 is illustrated in FIG. 3.In this exemplary embodiment, twenty reactor stages are used and arearranged for control purposes into two groups of ten stages each suchthat stages 301 through 319 are in a first group and stages 321 through341 are in a second group. Pre-reformed natural gas is introduced as areactant feed gas via line 343 and is analogous to the reactant feed gasin line 243 as described above with reference to FIG. 2. The reactantfeed gas flows via primary manifold 345 and is split to flow throughsecondary manifolds 347 and 349. Steam is fed into first stage 301 vialine 351.

The reactant feed gas flows through the reactant side of first stage301, and interstage reactant gas flows between successive stages throughstage 319 as described above with reference to FIG. 2. Interstagereactant gas from the first group of stages 301-319 flows via line 353and then flows through the reactant sides of the successive stages inthe second group of stages 321-341. Synthesis gas product flows from thesystem via line 355. In one embodiment, each of stages 301-341 may beenclosed in a separate pressure vessel 342, 344, 346, 348, 350, 352,354, 356, 358, 360, 362, 364, 366, 368, 370, 372, 374, 376, 378, and380. Alternatively, the first group of stages 301-319 may be installedin a single pressure vessel (not shown) wherein interstage reactant gasflows through open flow regions between stages and reactant gas frommanifold 347 is injected into the respective flow regions between thestages.

The reactant feed gas in manifold 347 is divided into ten individualstreams, and the first of these streams provides a reactant feed gas inline 357 that is mixed with the steam feed in line 351. The remainingnine reactant gas streams provide reactant interstage feed gas streamsthat are mixed with the corresponding interstage reactant gas streamsbetween pairs of adjacent stages 301 through 319 as shown. Similarly,the reactant feed gas in manifold 349 is divided into ten individualstreams to provide reactant interstage feed gas streams that are mixedwith the corresponding interstage reactant gas streams between pairs ofadjacent stages 321 through 341 as shown. The second group of stages321-341 may be installed in a single pressure vessel (not shown) whereininterstage reactant gas flows through open flow regions between stagesand reactant gas from manifold 349 is injected into the respective flowregions between the stages.

Manifold 347 may be designed to provide generally equal flow rates ofreactive gas into stages 301 to 319; alternatively, the manifold may bedesigned to provide a different flow rate to each stage or groups ofstages for reaction control purposes. Likewise, manifold 349 may bedesigned to provide generally equal flow rates of reactive gas intostages 321 to 341; alternatively, the manifold may be designed toprovide a different flow rate to each stage or groups of stages forreaction control purposes.

A first preheated oxidant gas is introduced via line 359 into manifold361 and is divided into ten portions to provide oxidant gas to theoxidant sides of stages 301 to 319 as shown. A second preheated oxidantgas is introduced via line 363 into manifold 365 and is divided into tenportions to provide oxidant gas to the oxidant sides of stages 321 to341 as shown. The first and second preheated oxidant gas streams may beprovided from a common upstream heater (not shown) and may be air heatedto a temperature of 600 to 1150° C. Oxygen permeates through themembranes in the stages and reacts with the reactive components on thereactant sides of the stages as described earlier above. Oxygen-depletednon-permeate gas is withdrawn from stages 301-319 via lines feeding intomanifold 367 and the gas is discharged via line 369. Similarly,oxygen-depleted non-permeate gas is withdrawn from stages 321-341 vialines feeding into manifold 371 and the gas is discharged via line 373.Heat and/or pressure energy may be recovered from the withdrawnnon-permeate gas by any of the known methods described in the art.

Manifold 361 may be designed to provide generally equal flow rates ofoxidant gas into stages 301 to 319; alternatively, the manifold may bedesigned to provide a different flow rate to each stage or groups ofstages, for example, for reaction and/or temperature control purposes.Likewise, manifold 365 may be designed to provide generally equal flowrates of oxidant gas into stages 321 to 341; alternatively, the manifoldmay be designed to provide a different flow rate to each stage or groupsof stages, for example, for reaction and/or temperature controlpurposes.

The temperatures in the stages and the reactions through the stages canbe regulated by controlling the flow rates of reactant and oxidant gasesto the stages and the operating conditions within the stages. This maybe accomplished, for example, by controlling the flow rates of reactantgas into manifolds 347 and 349 by control valves 375 and 377,respectively, and/or by controlling the flow rates of oxidant gas intomanifolds 361 and 365 by control valves 379 and 381, respectively. Forexample, control of the reactant gases to the group of stages 301 to 319may be effected by temperature indicator/controller 383, which sendscontrol signals via control line 385 to control valve 375. Likewise,control of the reactant gases to the group of stages 321 to 341 may beeffected by temperature indicator/controller 389, which sends controlsignals via control line 391 to control valve 377. In another example,temperature indicator/controller 383 may be located on oxygen-depletednon-permeate gas manifold 367 (not shown) and/or on oxygen-depletednon-permeate gas discharge line 369 (not shown). Likewise, temperatureindicator/controller 389 may be located on oxygen-depleted non-permeategas manifold 371 (not shown) and/or on oxygen-depleted non-permeate gasdischarge line 373 (not shown). Control of oxidant gases to the group ofstages 301 to 319 may be, for example, effected by control valve 379located on the oxidant gas stream upstream of the stages (as shown), orlocated on the oxygen-depleted non-permeate gas stream downstream of thestages (not shown). Likewise, control of oxidant gases to the group ofstages 321 to 341 may be, for example, effected by control valve 381located on the oxidant stream upstream of the stages (as shown), orlocated on the oxygen-depleted non-permeate gas stream downstream of thestages (not shown). In a typical embodiment, flow rates of oxidant gasesfed to the stages may be varied to maintain target oxygen concentrationin oxygen-depleted non-permeate gas withdrawn from the stages, forexample through manifolds 367 and 371. For example, oxygen concentrationmay be monitored by an oxygen analyzer/indicator controller (not shown)located on manifold 367 and/or discharge 369, which may send controlsignals via a control line (not shown) to control valve 379. Likewise,oxygen concentration may be monitored by an oxygen analyzer/indicatorcontroller (not shown) located on manifold 371 and/or discharge 373,which may send control signals via a control line (not shown) to controlvalve 381. Flow rate of permeated oxygen through the membranes may alsobe affected by the operating conditions (e.g. pressure and/ortemperature) within the stage, particularly in the vicinity of themembrane permeation surface. In another embodiment, any or all feedgases (e.g. reactant feed gas, reactant interstage feed gas, and/oroxidant gas) to a stage or series of stages may be omitted from orbypassed around the stage or series of stages to affect temperaturesand/or reactions.

The staged ITM reactor systems described above utilize reactant gasstreams that may include any of oxygen, steam, hydrocarbons,pre-reformed mixtures of steam and hydrocarbon feed gas, hydrogen,carbon monoxide, carbon dioxide, and/or carbon dioxide containing gas.Reactions that occur in the staged reactor systems may include, forexample, partial oxidation, complete oxidation, steam reforming, carbondioxide reforming, water-gas shift, and combinations thereof to producesynthesis gas. Certain of these reactions are strongly exothermic andothers are endothermic. Because ITM systems generally require a narrowoperating temperature range, proper control of the exothermic andendothermic reactions is required. The embodiments described aboveenable inherently stable operation in which the temperatures of themembrane system can be controlled within the required ranges.

The multiple reactant-staged membrane oxidation systems described aboveutilize at least two stages in series and may utilize up to 10 stages,up to 20 stages, up to 100 stages, or even greater than 100 stages inseries depending on specific operating and product requirements. Theembodiments are designed for the generation of synthesis gas, but may beapplied to any oxidation or partial oxidation processes utilizing oxygenprovided by permeation through ion transport membranes. When utilizedfor the generation of synthesis gas, the oxidant gas typically ispreheated air and steam is introduced into the reactant side of thefirst reactor stage. The reactant gas, for example pre-reformed naturalgas, is divided into multiple streams (not necessarily equal), which aresubsequently introduced into the multiple reactor stages. In someembodiments, it is preferable that the reactant gas in each stageapproach chemical equilibrium with respect to the steam reformingreaction, carbon dioxide reforming reaction, and/or the water gas shiftreaction when catalysts for these reactions are provided in each stage.The reactant gas entering a stage, exiting a stage, and/or exitingoptional catalyst (e.g. catalyst 1 d on FIG. 1) thus may be at or nearchemical equilibrium with respect to these reactions.

A flow of steam may be introduced into the reactant side of the firststage as shown in FIGS. 2 and 3 for the purpose of minimizingtemperature excursions and enabling the process to operate at or nearequilibrium in each of the stages. The term “temperature approach toequilibrium” for a given gas mixture is herein defined as the absolutevalue of the temperature difference between the actual temperature ofthe gas mixture and a calculated temperature at which the givenreactants in the gas mixture would be at chemical equilibrium.Temperature approach to equilibrium may be expressed with respect to thegas mixture as a whole or with respect to a specific reaction orreactions (e.g., steam reforming, carbon dioxide reforming, and/orwater-gas shift reactions) between specific reactants in the gasmixture. A typical temperature approach to equilibrium may be on theorder of 0 to 100° F., and typically may be on the order of 0 to 20° F.When the catalyst is disposed on or adjacent the membranes, thisapproach will occur within the modules of the stage; when the catalystis disposed following the membrane modules as shown in FIGS. 1 and 2,the approach to equilibrium will occur at the outlet of the catalystmodule. When catalyst is disposed preceding the membrane modules, theapproach to equilibrium will occur preceding the modules.

The temperature of the reactant side of the modules in each stage (andtherefore the temperature of the entire stage) may be controlled byvarying the amount and distribution of the reactant gas provided to eachstage. The reactant side of each stage is generally reactant-rich (e.g.,rich in oxidizable species) and the exothermic oxidation reactions whichgenerate temperature rise are limited by the rates of oxygen permeationthrough the membrane. The endothermic reforming reactions which generatetemperature drop are generally limited by both catalyst activity and bythe amount of hydrocarbon, particularly methane, fed to each stage.

By conducting the overall reaction in multiple stages, sufficientcatalyst may be provided within or between each of the stages such thatthe composition of the gas exiting a stage approaches equilibrium withrespect to the reactions occurring in that stage prior to entering thenext stage. This effectively tunes or regulates the catalyst activitysuch that endothermic reforming reactions and the associated temperaturedrop in each stage can be limited by the amount of hydrocarbon,particularly methane, fed to each stage as reactant interstage feed gas.Thus the hydrocarbon feed rate to each stage may be used to control thegas composition and temperature within and/or at the exit of the stage,and the composition and temperature of the gas can be made to approachequilibrium by controlling the hydrocarbon feed rate. A higherhydrocarbon feed rate will tend to cool the gas mixture via endothermicreforming, whereas a lower hydrocarbon feed rate will tend to yieldhigher temperatures by limiting the endothermic reforming that cools thereactant gas mixture.

If the oxidation reaction system is operated so that at least some ofthe reaction stages do not operate at or near equilibrium, varying thehydrocarbon feed rate to each stage also may be used to control the gascomposition and temperature within and/or at the exit of the stage. Ahigher hydrocarbon feed rate and feed concentration will lead toincreased rates of endothermic reforming reactions and thus tend to coolthe gas mixture, whereas a lower hydrocarbon feed rate and feedconcentration will lead to decreased rates of endothermic reformingreactions and thus tend to yield higher temperatures.

The membrane material used in the membrane oxidation reaction systemmust be operated within a specific temperature range. The uppertemperature limit is established based on criteria including the kineticdecomposition of the material, the mechanical creep of the membranes,the degree of iron volatilization in systems using iron-containingoxides, membrane stability in a synthesis gas environment in thepresence of volatilized iron, potential catalyst life issues, and othercriteria. The lower temperature limit is established based on thepotential for carbon dioxide-induced membrane degradation (discussedlater), phase decomposition, and other criteria. The embodimentsdescribed above provide a membrane-based oxidation reaction system whichprovides for inherently stable performance during steady-stateoperation, startup, shutdown, turndown, and recovery from process upsetconditions. This may be achieved by controlling the system within thecritical ranges of gas composition and temperature required for stableand reliable membrane operation.

Mixed conducting metal oxide membranes and membrane modules aremechanically fragile ceramic bodies. Mechanical stresses may accumulateas a result of temperature and composition gradients within the ceramicmaterial, and these stresses can damage the membranes as well as jointsand seals in the membrane modules. The use of reactant-staged membranereactors allows for the minimization of these thermal and concentrationgradients within the ceramic components of the reactor system.

Hydrocarbon-containing components in the feed gas may form elementalcarbon (soot) at the operating temperatures of ITM oxidation reactors,which may operate in a typical range of 600 to 1150° C. (1112 to 2102°F.). Undesirable formation of soot in the ITM modules, control valves,and internal piping will occur at these temperatures in certain reactivegas composition ranges. The potential for soot formation decreases astemperature decreases, and soot formation may be substantially lower inthe range 600 to 750° C. (1112 to 1382° F.) than at higher ITM operatingtemperatures of 850 to 1150° C. (1562 to 2102° F.). The potential forsoot formation decreases further and becomes readily manageable attemperatures in the range 500 to 650° C. (932 to 1202° F.) and below.Operation to avoid soot formation may be accomplished by utilizing thereactant-staged ITM system described above with appropriate control ofthe distribution, composition, and temperature of the reactant feed gasand reactant interstage feed gas among the stages.

The use of a reactant-staged ITM reactor system allows the introductionof the reactant gases at temperatures significantly below the operatingtemperatures of the ITM modules, which allows operation of the reactantgas feed system at lower temperatures. As a result, expensivehigh-temperature alloys may not be required for components of thereactant gas feed system.

Other potential problems in the operation of ITM oxidation reactorsystems may be minimized or eliminated by the use of the reactant-stagedembodiments described above. For example, oxygen flux may vary amongmembrane modules due to variations in manufacturing, installation,and/or operation. By staging the reactor system with respect to thereactant gases, control schemes may be utilized so that the reactorsystem can tolerate reasonable variations in oxygen flux among themultiple membrane modules in the system. Flux variations among themodules may be compensated for by controlling the flow and/orcomposition of the reactant interstage feed gas among the stages.Reactant-staged operation also can be used to address operating problemsassociated with startup, turndown, process upsets, and shutdown of ITMoxidation reactor systems.

The reactant-staged membrane oxidation reactor systems are illustratedabove for the generation of synthesis gas from natural gas. The systemsalternatively may be used for other oxidation or partial oxidationprocesses such as, for example, combustion (e.g. for power, steam, orheat generation) or reforming of hydrocarbons heavier than methane.

It is well known in the field of synthesis gas production that theinjection of CO₂ into the hydrocarbon feed to a steam-methane reformereffectively reduces the H₂/CO molar ratio in the synthesis gas product.This is often required to provide specific H₂/CO ratios in the synthesisgas feed to downstream chemical processes. CO₂ separated from thesynthesis gas product can be recycled to the reformer feed, or CO₂ canbe imported for this purpose from an outside source. For example,CO₂-containing offgas from a downstream Fischer-Tropsch (F-T)hydrocarbon synthesis process may be recycled to the feed of thesynthesis gas generation process to adjust the synthesis gas productH₂/CO molar ratio to about 2:1 as required for the F-T process. Thisexternal recycle step avoids the need for an acid gas removal system toremove CO₂ from the synthesis gas produced; instead, the F-T offgascontaining about 60 vol % CO₂ is recovered at high pressure along withother useful components for recycle, including light hydrocarbons, H₂,and CO. This external recycle step also serves to reduce CO₂ emissionswhile increasing the efficiency of carbon conversion into usefulproducts. Only a modest amount of compression is required to overcomepressure drop through the synthesis gas generation process and the F-Tprocess.

For an ITM oxidation process operated with an overall reactant feed gassteam-to-carbon (S/C) molar ratio of 1.5, which is desirable to minimizemethane slip (i.e. unreacted methane in the product synthesis gas) andcarbon deposition, the product synthesis gas H₂/CO molar ratio would beabout 3:1 (refer to Example 1 below). As described above, aFischer-Tropsch (F-T) reactor system generally requires a H₂/CO ratio ofabout 2:1. Other processes require even lower H₂/CO ratios; for example,certain oxo-alcohol synthesis processes require a 1:1 H₂/CO ratio in thesynthesis gas feed.

It has been found that in the production of synthesis gas by ITMoxidation reactors, high concentrations of CO₂ may react with anddecompose the material used in the membranes. The terms “decompose” or“decomposed” mean that the original membrane composition orstoichiometry is changed, for example by the reaction with CO₂.Exemplary membranes are complex multi-component metal oxides consistingof alkaline earth metals (e.g., Ca), transition metals, and lanthanum orlanthanides. The driving force for the reaction of these materials withCO₂ is the large negative free energy of formation of the alkaline earthcarbonates, for example, CaCO₃. A representative chemical reaction forthe decomposition of a perovskite in the presence of CO₂ can beexpressed in terms of the CO₂ partial pressure, p_(CO2), at equilibriumconditions, which may be defined as p_(CO2)*. At CO₂ partial pressuresless than p_(CO2)*, decomposition via CO₂ reactions will not occur andthe membrane material will be stable. At CO₂ partial pressures greaterthan p_(CO2)*, the membrane material will decompose via reaction withCO₂. The value of p_(CO2)* is a function of temperature, oxygen partialpressure, and membrane composition, and p_(CO2)* generally decreases astemperature decreases.

In addition, the membranes used in ITM oxidation reactors have an upperoperating temperature limit defined by other phenomena such as, forexample, kinetic decomposition, excessive material creep, reduction orvolatilization of transition metal species (e.g. Fe), and potentialcatalyst life issues.

Embodiments of the present invention may be applied to eliminate theproblem of membrane degradation by providing methods to recycle CO₂ inmembrane reactor systems for controlling the product synthesis gas H₂/COratio while ensuring that p_(CO2) remains less than p_(CO2)* throughoutthe membrane reactor system. The use of reactant-staged membraneoxidation reactor systems allows the controlled interstage introductionof carbon dioxide so that p_(CO2) remains less than p_(CO2)* in allstages. Lower carbon dioxide concentrations allow lower operatingtemperature limits, which in turn allow a larger operating temperaturewindow, thereby easing constraints on process control and providing amore operable process.

As described in detail below, p_(CO2)* is a strong function oftemperature, and the introduction of CO₂-containing gas into themembrane reactor system may be accomplished advantageously by dividingthe CO₂-containing gas into two or more reactant interstage feed gasstreams for injection between selected stages of a reactant-staged ITMoxidation system. The CO₂-containing gas may be injected into one ormore interstage points in the reactor system where higher temperatureslead to higher values of p_(CO2).

This embodiment is illustrated in the schematic flow diagram of FIG. 4.The exemplary reactant-staged membrane oxidation system comprises firststage 401, second stage 403, third stage 405, and last or n^(th) stage407. Each stage is illustrated schematically as a generic module havingan oxygen permeable membrane that divides the module into an oxidantside and a permeate or reactant side. As explained above, a stage cancomprise any number of membrane modules arranged in series and/orparallel and may include one or more catalysts selected from oxidationcatalyst, steam reforming catalyst, carbon dioxide reforming catalyst,and water gas shift catalyst. Any desired number of stages may be usedas long as there are at least two stages.

First stage 401 comprises oxidant side 401 a, membrane 401 b, reactantside 401 c, optional catalyst 401 d, and appropriate gas inlet andoutlet regions. Optional catalyst 401 d is shown here as immediatelyfollowing the module, but alternatively or additionally catalyst may bedisposed immediately preceding the module (not shown) or within oraround the module in any desired configuration (not shown). Similarly,second stage 403 comprises oxidant side 403 a, membrane 403 b, reactantside 403 c, appropriate gas inlet and outlet regions, and optionalcatalyst 403 d, which is shown here as immediately following the module.Alternatively or additionally, catalyst may be disposed immediatelypreceding the module (not shown) or within or around the module in anydesired configuration (not shown). Similarly, third stage 405 comprisesoxidant side 405 a, membrane 405 b, reactant side 405 c, appropriate gasinlet and outlet regions, and optional catalyst 405 d. Optional catalyst405 d is shown here as immediately following the module, butalternatively or additionally catalyst may be disposed immediatelypreceding the module (not shown) or within or around the module in anydesired configuration (not shown). Last stage 407 comprises oxidant side407 a, membrane 407 b, reactant side 407 c, appropriate gas inlets andoutlets, and optional catalyst 407 d, shown here as immediatelyfollowing the module. Alternatively or additionally, catalyst may bedisposed immediately preceding the module (not shown) or within oraround the module in any desired configuration (not shown). Product gasfrom last or n^(th) stage 407 is withdrawn via product line 407 e.Interstage reactant gas flows from stage 401 via line 401 e, from stage403 via line 403 e, and from stage 405 via line 405 e.

An oxidant gas, for example, preheated air or oxygen-containingcombustion products from a combustor operated with excess air, isintroduced via line 409 into oxidant side 401 a of first stage 401 andcontacts the oxidant side of membrane 401 b, a portion of the oxygenpermeates through membrane 401 b, and oxygen-depleted gas exits firststage 401 via line 411. Similarly, additional oxidant gas streams may beintroduced via lines 413, 415, and 419 into stages 403, 405, and 407,respectively, and oxygen-depleted gas may exit the stages via lines 421,423, and 425, respectively.

A reactant gas feed such as, for example, pre-reformed natural gas isprovided via line 427 and is mixed with steam provided via line 429, andthe mixed reactant gas feed is introduced into reactant side 401 c ofstage 401. Another gas stream comprising reactant components is providedvia line 431 from a source different than the source of the reactant gasfeed provided via line 427. The gas in line 431 may be, for example,offgas from a downstream process that utilizes synthesis gas produced bythe staged oxidation reactor system of FIG. 4. The downstream processmay be a hydrocarbon synthesis process (e.g., a Fischer-Tropsch process)or an oxygenated hydrocarbon synthesis process (e.g., an alcoholsynthesis process). In one embodiment described in more detail below,the gas in line 431 is offgas from a Fischer-Tropsch hydrocarbonsynthesis reactor system that contains a major concentration of CO₂,smaller concentrations of H₂ and CO, and some unreacted CH₄. This gas inline 431 optionally may be combined with steam provided via line 433,the mixture heated in pre-heater 435, and the heated mixture introducedinto steam-methane reformer 437 in which some of the unreacted methaneis converted to additional H₂ and carbon oxides. In some situations, itmay not be necessary to reform the offgas before recycle, and reformer437 would not be required.

The partially reformed offgas stream flows via manifold 439, is dividedinto separate streams in lines 441, 443, and 445, and these steams areoptionally heated in heat exchangers 447, 449, and 451 to providereactant interstage feed gas in lines 453, 455, and 457. The reactantgas feed in reactant side 401 c of first stage 401 reacts with permeatedoxygen, reforming and/or shift reactions may be promoted by optionalcatalyst 401 d, and the first stage effluent flows via flow path 401 e.Reactant interstage feed gas is provided via line 453, is mixed withfirst stage effluent in flow path 401 e, and the mixed interstagereactant gas flows into reactant side 403 c of second stage 403.Alternatively, the reactant interstage feed gas in line 453 may beintroduced directly into the reactant side of stage 403 and/or upstreamof catalyst 401 d.

The gas in reactant side 403 c of second stage 403 reacts with permeatedoxygen, reforming and/or shift reactions may be promoted by optionalcatalyst 403 d, and the second stage effluent flows via flow path 403 e.Reactant interstage feed gas is provided via line 455, is mixed withsecond stage effluent in flow path 403 e, and the mixed interstagereactant gas flows into reactant side 405 c of third stage 405.Alternatively, the reactant interstage feed gas in line 455 may beintroduced directly into the reactant side of stage 405 and/or upstreamof catalyst 403 d.

The gas in reactant side 405 c of third stage 405 reacts with permeatedoxygen, reforming and/or shift reactions may be promoted by optionalcatalyst 405 d, and the third stage effluent flows via flow path 405 e.Reactant interstage feed gas is provided via line 457, is mixed withthird stage effluent in flow path 405 e, and the mixed interstagereactant gas flows into reactant side 407 c of last stage 407.Alternatively, the reactant interstage feed gas in line 457 may beintroduced directly into the reactant side of stage 407 and/or upstreamof catalyst 405 d.

The gas in reactant side 407 c of last stage 407 reacts with permeatedoxygen, reforming and/or shift reactions may be promoted by optionalcatalyst 407 d, and the last stage effluent flows via line 407 e toprovide a synthesis gas product. The product gas may be at a temperatureof 600 to 1150° C. and a pressure of 2 to 40 atma, and the gas typicallycomprises hydrogen, carbon monoxide, water, carbon dioxide, and methane.Any number of additional stages may be utilized between third stage 405and last stage 407 if desired.

In the illustration of FIG. 4 described above, interstage reactant gasflows from stage 401 via flow path 401 e, from stage 403 via flow path403 e, and from stage 405 via flow path 405 e. In one embodiment, eachof stages 401, 403, 405, and 407 may be enclosed in a separate pressurevessel 412, 414, 416, and 418; in this case, flow paths 401 e, 403 e,and 405 e are pipes, conduits, or closed channels between the vessels.In another embodiment, stages 401, 403, 405, and 407 may be enclosed ina single pressure vessel (not shown) such that reactant gas can flowthrough the reactant zones of each stage in succession; in this case,flow paths 401 e, 403 e, and 405 e are open regions between stagesthrough which gas can flow from the reactant gas outlet region of onestage into the reactant gas inlet region of the following stage.

The embodiment of FIG. 4 is particularly useful when CO₂-rich gas (forexample, recycle gas from an external process) is utilized for thepurpose of controlling the H₂/CO ratio in the product synthesis gas. Asmentioned above, it has been found that the mixed conducting metal oxidematerials used in the membranes may react with CO₂ and degrade ordecompose when the CO₂ partial pressure in the reactive gas, p_(CO2), isgreater than a critical threshold partial pressure defined as p_(CO2)*.The CO₂ partial pressures in the reactor stages therefore should becontrolled as described below to minimize or eliminate membranedegradation.

In the process of FIG. 4, a steam-to-carbon (S/C) molar ratio of about1.5 in the reactant feed gas is desirable to minimize methane slip andcarbon deposition in the system. With this reactant feed gas, theproduct synthesis gas H₂/CO molar ratio would be about 3:1. Theinjection of the CO₂-rich gas stream into the reactant feed gas to firststage 401 would effectively reduce the product H₂/CO ratio to a levelrequired for downstream chemical processes. For example, this ratioshould be about 2:1 for a Fischer-Tropsch hydrocarbon synthesis process.At CO₂ partial pressures greater than the critical threshold, p_(CO2)*,however, the membrane material may react with the CO₂ and decompose.This can be avoided by operating the membrane reactor system so thatp_(CO2) is less than p_(CO2)* throughout the system, thereby enhancingthe operating life of the membrane. It may not be possible to maintainp_(CO2) less than p_(CO2)* when all of the recycled CO₂ is introducedinto the reactant gas feed to the first stage.

FIG. 5 shows the behavior of p_(CO2)* as a function of temperature andoxygen partial pressure for a typical material used in ITM reactorsystems, La_(0.9)Ca_(0.1)FeO_(3-δ), wherein the value of δ makes thecompound charge neutral. This plot was generated from thermodynamiccalculations of the complex phase equilibria exhibited by thesematerials. The oxygen partial pressure, p_(O2), of the synthesis gasstream is itself determined from thermodynamic calculations rather thanmeasured directly; it is also a strong function of temperature andsynthesis gas composition.

Because p_(CO2)* is a strong function of temperature, CO₂ injection isproblematic at lower temperatures typical of the inlet conditions to amembrane reactor system. Since the synthesis gas generation process isnet exothermic and the reactant gas temperature increases down thelength of the reactor, it can be advantageous to inject the CO₂ at oneor more intermediate points in the reactor system where highertemperature leads to much higher values of p_(CO2)*. This is especiallybeneficial when the CO₂ injection stream is significantly cooler thanthe reactant feed stream to the membrane reactor and the injectionitself causes a substantial drop in temperature. In this case, dividingthe CO₂ injection stream into multiple intermediate injections asdescribed above can mitigate the temperature drop at each injectionpoint. A significant temperature drop is generally undesirable becauseit creates a locally depressed value of p_(CO2)* and also leads tosignificant thermal stress in the membranes.

As pointed out above, maintaining p_(CO2)<p_(CO2)* throughout the systemduring operation is beneficial to long-term membrane life in an ITMreactor system. FIG. 5, which is a plot of p_(CO2)* vs. temperature atvarious equilibrium O₂ partial pressures for the mixed conducting metaloxide membrane material La_(0.9)Ca_(0.1)FeO_(3-δ), shows that this taskis most difficult at lower temperatures typical of the inlet conditionsto an ITM synthesis gas generation process. Since the oxidationreactions in the process are net exothermic and the reactant gastemperature increases down the length of the reactor, injection ofCO₂-containing gas is best accomplished at one or more intermediatepoints in the staged reactor system where the higher temperature andcorresponding oxygen partial pressure lead to much higher values ofp_(CO2)*. As long as the cooling effects are mitigated properly, thecooling provided by CO₂-containing gas will be beneficial to the processbecause of the upper operating temperature limit of the membranesimposed by other material requirements.

Cooling provided by CO₂-containing gas follows several mechanisms, allof which are most beneficial when occurring in a higher temperature zonedownstream from the inlet to the membrane reactor. Obviously, theCO₂-containing gas stream can provide sensible cooling. In addition, thepresence of CO₂ leads to the endothermic reverse shift reaction and theendothermic CO₂ reforming reaction; when these reactions occur on ornear the membrane surfaces, they provide beneficial membrane cooling.Furthermore, any hydrocarbon components present in the CO₂-containinggas stream may undergo the endothermic reforming reaction on themembranes, leading to further cooling. Finally, intermediateinjection(s) of CO₂-containing gas leads to less mass flow and lowerpressure drop at the front end of the membrane reactor.

The embodiment described above illustrates the use of ITM reactorsystems for the production of synthesis gas with controlled H₂/COratios. The embodiment may be applied to any partial oxidation reactionwith introduction of a CO₂-containing stream.

The following Examples illustrate embodiments of the present inventionbut do not limit embodiments of the invention to any of the specificdetails described therein.

Example 1

An embodiment of the invention similar to FIGS. 2 and 3 was simulatedusing the process simulator Aspen Plus™ from Aspen Technology, Inc. Thesimulation utilized 100 membrane reactor stages in series wherein thereactant gas feed is divided into ten portions (not necessarily equal)and each portion is divided into ten equal sub-portions. Eachsub-portion is fed to the inlet of a corresponding stage wherein thesub-portion to the first stage is mixed with steam and each of theremaining sub-portions is provided as a reactant interstage feed gasthat is mixed with a corresponding interstage reactant gas stream.

The following specific process features and parameters were used in thesimulation:

-   -   Total sum of oxygen permeation in all stages is 1000 kgmol/hr        with an evenly-distributed oxygen flux of 10 kgmol/hr in each        stage.    -   Overall steam-to-carbon molar ratio is 1.5 in the reactant feed        to the system wherein the steam-to-carbon ratio is defined as        total water divided by total organic carbon in the stream;        carbon contained in carbon dioxide and carbon monoxide is not        included.    -   Natural gas feed has the following composition: 94.73% methane,        3.16% ethane, 0.54% propane, 0.18% butane, 0.06% pentane, 0.04%        hexane, 0.71% carbon dioxide, 0.58% nitrogen (compositions in        mole %).    -   A small amount of hydrogen is added to the natural gas, as is        typical for desulfurization, e.g. approximately 3% of the total        carbon feed on a molar basis.    -   Reactant gas feed is provided by pre-reforming natural gas in an        adiabatic pre-reformer with an inlet temperature of 510° C. to        convert hydrocarbons heavier than methane.    -   Reactant gas pressure on the reactant sides of all stage        membrane modules is 30.3 bara (440 psia) and the oxidant gas        pressure on the oxidant sides of all stage membrane modules is        1.7 bara (25 psia).

TABLE 1 Reactant Feed Distribution for Example 1 Flow to StageDistribution of Pre-reformed Group Reactant Feed to Stage Groups Portion(kgmol/hr) (%) Stages 1 through 10 1 332.2 9.6 Stages 11 through 20 2330.7 9.5 Stages 21 through 30 3 342.4 9.9 Stages 31 through 40 4 358.710.3 Stages 41 through 50 5 374.5 10.8 Stages 51 through 60 6 388.1 11.2Stages 61 through 70 7 399.2 11.5 Stages 71 through 80 8 408.2 11.7Stages 81 through 90 9 415.6 12.0 Stages 91 through 100 10 125.2 3.6Total to all stages 3474.9 100.0

Natural gas at 2142.9 kgmol/hr, 67.3 kgmol/hr hydrogen, and 1113.5kgmol/hr steam are mixed and heated to 510° C. The heated mixture ispre-reformed in an adiabatic pre-reformer reactor and exits theadiabatic pre-reformer reactor at 474° C. The pre-reformed mixture isdivided into the portions given in Table 1.

Each of portions 1-10 in Table 1 is divided into ten equal sub-portions.Steam at 2227.1 kgmol/hr is preheated to 875° C. and mixed with a firstsub-portion of portion 1, and the mixture is introduced to the reactantside of the first stage. The other 99 sub-portions (all of thesub-portions except the first sub-portion of portion 1) are provided asreactant interstage feed gas streams, each of which is mixed with arespective interstage reactant gas stream and fed to the inlet of eachof the respective 99 stages. Air is provided to the oxidant side of eachstage at 855° C., and oxygen-depleted air is withdrawn from the oxidantside of each stage.

Sufficient catalyst is provided in this Example on the reactant sides ofall membrane modules such that the steam reforming, carbon dioxidereforming, and water-gas shift reactions occur within the modules andmaintain essentially chemical equilibrium conditions throughout the100-stage reactor system. A final synthesis gas product is withdrawnfrom stage 100 at 900° C. and 9442.2 kgmol/hr with a composition of46.7% hydrogen, 3.0% methane, 6.7% carbon dioxide, 14.1% carbonmonoxide, 29.4% water, and 0.1% nitrogen (compositions in mole %).

A summary of the simulation results is given in Table 2 showing thereactant gas temperature and composition from the first reactor stageand from each succeeding group of 10 stages. FIG. 6 shows thetemperature of the reactant gas at the inlet and outlet of each stageplotted as a percentage of the group of 100 reaction stages from thereactant feed inlet (0) to the product outlet (100). It is seen thatstaged operation of the reactor system controls the reactant gastemperature within the preferred membrane temperature range of 850 to900° C. The temperature of the reactant gas is generally flat at about875° C. through the first 90 stages and rises to 900° C. in the last 10stages. The saw-tooth temperature profile is attributed to quenching ofthe reactant gas by the introduction of cold (474° C.) pre-reformedreactant feed gas via the reactant interstage feed lines into theinterstage reactant gas between each of the 100 stages.

TABLE 2 Simulation Summary for Example 1 Pre- ITM Total Ref'r Reactor O₂Reactant Gas at Outlet of Reactor Stage No. Feed Feed Permeate 1 10 2030 40 50 60 70 80 90 100 Temp., ° C. 510 474 874 875 875 874 875 875 875875 875 900 900 Press., bara 33.3 32.7 30.3 30.3 30.3 30.3 30.3 30.330.3 30.3 30.3 30.3 30.3 Flow, 3324 3475 1000 2301.4 2969.5 3696.04424.5 5162.2 5910.2 6667.7 7433.2 8205.2 8982.7 9442.2 kgmol/hr H₂, mol% 2.02 7.31 2.78 19.91 29.75 35.44 39.00 41.36 42.98 44.13 44.97 45.5946.69 CH₄, mol % 61.08 61.91 0.00 0.02 0.20 0.60 1.14 1.76 2.40 3.013.59 4.13 2.98 CO₂, mol % 0.46 2.50 0.90 5.38 6.99 7.49 7.58 7.52 7.397.25 7.10 6.96 6.68 CO, mol % 0.11 0.03 1.82 4.38 6.58 8.33 9.70 10.8011.67 12.38 12.97 14.09 H₂O, mol % 33.50 27.80 96.28 72.83 58.62 49.8243.85 39.55 36.32 33.82 31.83 30.21 29.43 N₂, mol % 0.37 0.36 0.01 0.040.06 0.08 0.09 0.11 0.11 0.12 0.13 0.13 0.13 O₂, mol % 100 C₂H₆, mol %2.04 C₃H₈, mol % 0.35 C₄H₁₀, 0.12 mol % C₅H₁₂, 0.04 mol % C₆H₁₄, 0.03mol %

Example 2

Operation of the system of Example 2 was simulated using the samesimulation method and the same specific process features and parametersas in Example 1, except that catalyst is not used within the membranemodules. Instead, an adiabatic catalyst bed is provided at the exit ofeach stage (see FIGS. 1 and 2) to equilibrate the steam reforming,carbon dioxide reforming, and water-gas shift reactions. 2219.7 kgmol/hrsteam is preheated to 860° C. and introduced to the first reactor stage.2135.8 kgmol/hr natural gas, 67.0 kgmol/hr hydrogen, and 1109.9 kgmol/hrsteam are mixed and preheated to 510° C., pre-reformed in an adiabaticpre-reformer reactor, and withdrawn from the adiabatic pre-reformerreactor at 474° C. The pre-reformed mixture is divided into the portionsgiven in Table 3.

TABLE 3 Reactant Feed Gas Distribution for Example 2 Flow to StageDistribution of Pre-reformed Group Reactant Feed to Stage Groups Portion(kgmol/hr) (%) Stages 1 through 10 1 335.1 9.7 Stages 11 through 20 2327.8 9.5 Stages 21 through 30 3 339.2 9.8 Stages 31 through 40 4 352.110.2 Stages 41 through 50 5 362.9 10.5 Stages 51 through 60 6 372.4 10.8Stages 61 through 70 7 379.1 10.9 Stages 71 through 80 8 385.6 11.1Stages 81 through 90 9 390.1 11.3 Stages 91 through 100 10 219.2 6.3Total to all stages 3463.4 100.0

Each portion in Table 3 is divided into ten equal sub-portions. Thepreheated steam is mixed with a first sub-portion of portion 1, and themixture is introduced to the reactant side of the first stage. The other99 sub-portions (all of the sub-portions except the first sub-portion ofportion 1) are provided as reactant interstage feed gas streams, each ofwhich is mixed with a respective interstage reactant gas stream and fedto the inlet of each of the respective 99 stages. Air is provided to theoxidant side of each stage at 855° C., and oxygen-depleted air iswithdrawn from the oxidant side of each stage. A final synthesis gasproduct is withdrawn from stage 100 at 900° C. and 9413.1 kgmol/hr witha composition of 46.7% hydrogen, 3.0% methane, 6.7% carbon dioxide,14.1% carbon monoxide, 29.5% water, and 0.1% nitrogen (compositions inmole %).

A summary of the simulation results is given in Table 4 showing thereactant gas temperature and composition from the first reactor stageand from each succeeding group of 10 stages. FIG. 7 shows thetemperature of the reactant gas at the inlet and outlet of each stageplotted as a percentage of the group of 100 reaction stages from thereactant feed inlet (0) to the product outlet (100). It is seen that thetemperature of the reactant gas is within the desired control range of850 to 900° C. through the first 90 stages. The temperature of thereactant gas rises above 900° C. in the last 10 stages and exits thelast stage at 900° C. The saw-tooth temperature profile is attributed tothe exothermic partial and complete oxidation reactions occurring ineach stage followed by net endothermic equilibration of the steamreforming, carbon dioxide reforming, and water-gas shift reactions ineach adiabatic catalyst bed following the stage.

TABLE 4 Simulation Summary for Example 2 Pre- ITM Total Ref'r Reactor O₂Reactant Gas at Outlet of Reactor Stage No. Feed Feed Permeate 1 10 2030 40 50 60 70 80 90 100 Temp., ° C. 510 474 858 860 867 872 875 877 879881 882 883 900 Press., bara 33.3 32.7 30.3 30.3 30.3 30.3 30.3 30.330.3 30.3 30.3 30.3 30.3 Flow, 3313 3463 1000 2294.7 2968.2 3686.44407.3 5134.6 5868.0 6607.7 7351.5 8100.1 8851.7 9413.1 kgmol/hr H₂, mol% 2.02 7.31 2.82 20.16 29.77 35.35 38.88 41.25 42.92 44.14 45.06 45.7646.65 CH₄, mol % 61.08 61.91 0.00 0.03 0.23 0.63 1.13 1.66 2.19 2.683.14 3.56 2.97 CO₂, mol % 0.46 2.50 0.91 5.47 7.05 7.53 7.60 7.52 7.387.23 7.07 6.93 6.69 CO, mol % 0.11 0.03 1.79 4.32 6.52 8.28 9.70 10.8411.76 12.53 13.17 14.08 H₂O, mol % 33.50 27.80 96.23 72.51 58.56 49.8944.00 39.76 36.56 34.07 32.08 30.46 29.47 N₂, mol % 0.37 0.36 0.01 0.040.06 0.08 0.09 0.10 0.11 0.12 0.13 0.13 0.13 O₂, mol % 100 C₂H₆, mol %2.04 C₃H₈, mol % 0.35 C₄H₁₀, 0.12 mol % C₅H₁₂, 0.04 mol % C₆H₁₄, 0.03mol %

Example 3

Operation of the system of Example 3 was simulated using the samesimulation method and the same specific process features and parametersas in Example 1. 1505.3 kgmol/hr natural gas, 47.3 kgmol/hr hydrogen,and 782.2 kgmol/hr steam are mixed and preheated to 510° C. The heatedmixture is pre-reformed in an adiabatic pre-reformer reactor and exitsthe adiabatic pre-reformer reactor at 474° C. A stream of recycledoffgas from a Fischer-Tropsch synthesis reactor at 2631.5 kgmol/hr and38° C. having a composition of 38.1% hydrogen, 11.7% methane, 28.8%carbon dioxide, 19.10% carbon monoxide, 0.3% water, and 2.0% nitrogen ismixed with the pre-reformed reactant gas. The resulting mixed reactantfeed gas at 282° C. is divided and distributed to the 100 ITM stages fortemperature control, and the amount of recycle gas is used to controlthe H₂/CO ratio in the synthesis gas product from the reactor system.The mixed reactant feed gas is divided into portions as given in Table5.

Each portion in Table 5 is divided into ten equal sub-portions. 1564.5kgmol/hr of steam is preheated to 875° C., mixed with the firstsub-portion of portion 1, and the mixture is introduced to the reactantside of the first stage. The other 99 sub-portions (all of thesub-portions except the first sub-portion of portion 1) are provided asreactant interstage feed gas streams, each of which is mixed with arespective interstage reactant gas stream and fed to the inlet of eachof the respective 99 stages. Air is provided to the oxidant side of eachstage at 855° C. and oxygen-depleted air is withdrawn from the oxidantside of each stage.

Sufficient catalyst is provided in this Example on the reactant sides ofall membrane modules such that the steam reforming, carbon dioxidereforming, and water-gas shift reactions occur within the modules andmaintain essentially chemical equilibrium conditions throughout the100-stage reactor system. A final synthesis gas product is withdrawnfrom stage 100 at 900° C. and 9702.2 kgmol/hr with a composition of40.3% hydrogen, 3.0% methane, 9.8% carbon dioxide, 19.7% carbonmonoxide, 26.7% water, and 0.6% nitrogen (compositions in mole %).

A summary of the simulation results is given in Table 6 showing thereactant gas temperature and composition from the first reactor stageand from each succeeding group of 10 stages. FIG. 8 shows thetemperature of the reactant gas at the inlet and outlet of each stageplotted as a percentage of the group of 100 reaction stages from thereactant feed inlet (0) to the product outlet (100). It is seen thatstaged operation of the reactor system controls the reactant gastemperature within the preferred membrane temperature range of 850 to900° C. The temperature of the reactant gas rises from nominally 870 to900° C. in the last 10 stages. The saw-tooth temperature profile isattributed to quenching of the interstage reactant gas upon introductionof cold mixed reactant feed gas (i.e., pre-reformed feed gas plusrecycle gas) into the interstage reactant gas between each of the 100stages.

TABLE 5 Mixed Reactant Feed Gas Distribution for Example 3 Distributionof Mixed Flow to Stage Reactant Feed Gas to Group Stage Groups Portion(kgmol/hr) (%) Stages 1 through 10 1 492.9 9.7 Stages 11 through 20 2497.1 9.8 Stages 21 through 30 3 522.0 10.3 Stages 31 through 40 4 543.410.7 Stages 41 through 50 5 558.6 11.0 Stages 51 through 60 6 569.0 11.2Stages 61 through 70 7 576.4 11.4 Stages 71 through 80 8 581.6 11.5Stages 81 through 90 9 585.5 11.5 Stages 91 through 100 10 145.9 2.9Total to all stages 5072.5 100.0

TABLE 6 Simulation Summary for Example 3 Pre- ITM Total Ref'r Reactor O₂Reactant Gas at Outlet of Reactor Stage No. Feed Feed Permeate 1 10 2030 40 50 60 70 80 90 100 Temp., ° C. 510 282 875 875 875 875 875 875 875875 875 875 900 Press., bara 33.3 32.7 30.3 30.3 30.3 30.3 30.3 30.330.3 30.3 30.3 30.3 30.3 Flow, 2335 5073 1000 1649.1 2407.0 3230.64065.1 4912.1 5768.5 6631.6 7499.4 8370.6 9244.4 9702.2 kgmol/hr H₂, mol% 2.02 23.28 3.98 23.58 31.50 35.07 36.90 37.92 38.53 38.90 39.15 39.3140.30 CH₄, mol % 61.08 35.86 0.00 0.08 0.53 1.18 1.85 2.47 3.01 3.473.87 4.22 2.95 CO₂, mol % 0.46 16.15 1.76 8.64 10.31 10.69 10.74 10.6710.58 10.48 10.39 10.30 9.79 CO, mol % 9.96 0.09 3.96 8.16 11.18 13.3414.94 16.16 17.11 17.88 18.50 19.66 H₂O, mol % 33.5 13.54 94.13 63.4849.13 41.43 36.67 33.45 31.15 29.42 28.09 27.02 26.67 N₂, mol % 0.371.21 0.04 0.25 0.37 0.45 0.51 0.55 0.58 0.61 0.63 0.64 0.63 O₂, mol %100 C₂H₆, mol % 2.04 C₃H₈, mol % 0.35 C₄H₁₀, 0.12 mol % C₅H₁₂, 0.04 mol% C₆H₁₄, 0.03 mol %

Example 4

Consider an ITM oxidation reactor system with membranes having acomposition that exhibits the p_(CO2)* behavior characterized by FIG. 5.The oxidation system is integrated with a Fischer-Tropsch (F-T) processwherein the F-T offgas is recycled to the ITM reactor system to adjustthe synthesis gas product H₂/CO molar ratio to about 2:1. In thisexample, a single stage reactor system operates at 30 bara and 800° C.Sufficient catalyst is provided in this Example on the reactant sides ofall membrane modules such that the steam reforming, carbon dioxidereforming, and water-gas shift reactions occur within the modules andmaintain essentially chemical equilibrium conditions throughout thereactor system. The F-T offgas is mixed with natural gas feed and thecombined feed is mixed with steam before being partially reformed in apre-reformer. This pre-reformed gas provides reactant feed gas to thesingle stage ITM reactor system. The reactor inlet gas composition is5.5% H₂, 0.4% CO, 16.3% CO₂, 46.6% H₂O, 30% CH₄, and 1.3% N₂(compositions in mole %). At these feed conditions, the equilibriumpartial pressure of oxygen is 2.5×10⁻¹⁹ bara and p_(CO2)* is 1.7 bara(from FIG. 5). However, the actual equilibrium value of p_(CO2) is 3.2bara, well above p_(CO2)*, indicating that decomposition of the reactormembranes would occur.

Example 5

The ITM oxidation reactor system of FIG. 4 is operated with two stages401 and 403, and stages 405-407 and manifold 439 are not used.Sufficient catalyst is provided in this Example on the reactant sides ofall membrane modules such that the steam reforming, carbon dioxidereforming, and water-gas shift reactions occur within the modules andmaintain essentially chemical equilibrium conditions throughout thereactor system. Pre-reformed natural gas in line 427 is mixed with steamvia line 429 and the mixture flows via line 430 into first reactor stage401. The reactant stream is partially converted in stage 401 and reachesequilibrium at the upper operating temperature limit of 900° C. at theoutlet of stage 401. F-T offgas via line 431 is preheated in heatexchanger 435 and is reformed in pre-reformer 437 to convert heavierhydrocarbons in the F-T offgas. Steam is added via line 433. Thepre-reformed gas at 560° C. flows via lines 441 and 453 (heater 447 isnot used) and is mixed with the interstage reactant gas in line 401 e.The mixed gas flows via line 402 into second reactor stage 403. Selectedoperating conditions and gas compositions are given in Table 7.

TABLE 7 Operating Conditions for Example 5 Interstage feed toPre-reformed feed stage 403 (line 430) (line 402) Pressure (bara) 30 26Temperature (° C.) 800 783 H₂ (mol %) 8.0 36.1 CO (mol %) 0.1 11.4 CO₂(mol %) 3.2 16.4 H₂O (mol %) 52.6 28.8 CH₄ (mol %) 35.7 6.3 N₂ (mol %)0.4 1.1

The equilibrium partial pressure of oxygen at the conditions in line 430is 1.5×10⁻¹⁹ bara, p_(CO2)* is 1.65 bara (from FIG. 5), and the actualequilibrium value of p_(CO2) is 1.61 bara. Thus, the membranes areessentially at the limit of decomposition stability via CO₂ reaction atthe inlet to stage 401. In line 402 at the inlet to second stage 403,the equilibrium partial pressure of oxygen is 1.9×10⁻¹⁹ bara andp_(CO2)* is 1.5 bara (from FIG. 5). The actual equilibrium value ofp_(CO2), however, is 3.9 bara, indicating that decomposition of themembranes in stage 403 would occur.

Example 6

The ITM oxidation reactor system of FIG. 4 is operated with three stages401, 403, and 405. Stage 407 is not used. Sufficient catalyst isprovided in this Example on the reactant sides of all membrane modulessuch that the steam reforming, carbon dioxide reforming, and water-gasshift reactions occur within the modules and maintain essentiallychemical equilibrium conditions throughout the reactor system.Pre-reformed natural gas in line 427 is mixed with steam via line 429and the mixture flows via line 430 into first reactor stage 401. Thereactant stream is partially converted in stage 401 and reachesequilibrium at the upper operating temperature limit of 900° C. at theoutlet of stage 401. F-T offgas via line 431 is preheated in heatexchanger 435 and is reformed in pre-reformer 437 to convert heavierhydrocarbons in the F-T offgas. Steam is added via line 433. Half of thepre-reformed offgas at 560° C. flows via lines 441 and 453 (heater 447is not used) and is mixed with the interstage reactant gas in line 401e. The mixed gas flows via line 402 into second reactor stage 403, thereactant stream is partially converted therein, and the stream reachesequilibrium at the upper operating temperature limit of 900° C. at theoutlet of stage 403. The other half of the pre-reformed offgas at 560°C. flows via lines 443 and 455 (heater 449 is not used) and is mixedwith the interstage reactant gas in line 403 e. The mixed interstage gasflows via line 404 into third reactor stage 405.

Selected operating conditions and gas compositions are given in Table 8.

TABLE 8 Operating Conditions for Example 6 Interstage feed toPre-reformed feed stage 403 (line 430) (line 402) Pressure (bara) 30 28Temperature (° C.) 800 829 H₂ (mol %) 8.0 41.1 CO (mol %) 0.1 12.5 CO₂(mol %) 3.2 12.3 H₂O (mol %) 52.6 28.1 CH₄ (mol %) 35.7 5.3 N₂ (mol %)0.4 0.8

Conditions at the inlet to first stage 401 are similar to those inExample 5 (above). The equilibrium partial pressure of oxygen is1.2×10⁻¹⁸ bara in line 402 at the inlet to stage 403 and p_(CO2)* is 2.9bara (from FIG. 5). Since the actual equilibrium value of p_(CO2) is 3.0bara, however, decomposition of the membranes in stage 403 would occur.

Example 7

The ITM oxidation reactor system of FIG. 4 is operated with four stages401, 403, 405, and 407. Sufficient catalyst is provided in this Exampleon the reactant sides of all membrane modules such that the steamreforming, carbon dioxide reforming, and water-gas shift reactions occurwithin the modules and maintain essentially chemical equilibriumconditions throughout the reactor system. Pre-reformed natural gas inline 427 is mixed with steam via line 429 and the mixture flows via line430 into first reactor stage 401. The reactant stream is partiallyconverted in stage 401 and reaches equilibrium at the upper operatingtemperature limit of 900° C. at the outlet of stage 401. F-T offgas vialine 431 is preheated in heat exchanger 435 and is reformed inpre-reformer 437 to convert heavier hydrocarbons in the F-T offgas.Steam is added via line 433.

One-third of the pre-reformed offgas at 560° C. flows via lines 441 and453 (heater 447 is not used) and is mixed with the interstage reactantgas in line 401 e. The mixed gas flows via line 402 into second reactorstage 403, the reactant stream is partially converted therein, and thestream reaches equilibrium at the upper operating temperature limit of900° C. at the outlet of stage 403. Another one-third of thepre-reformed offgas at 560° C. flows via lines 443 and 455 (heater 449is not used) and is mixed with the interstage reactant gas in line 403e. The mixed interstage gas flows via line 404 into third reactor stage405, the reactant stream is partially converted therein, and the streamreaches equilibrium at the upper operating temperature limit of 900° C.at the outlet of stage 405.

The remaining one-third of the pre-reformed offgas at 560° C. flows vialines 445 and 457 (heater 451 is not used) and is mixed with theinterstage reactant gas in line 405 e. The mixed interstage gas flowsvia line 406 into fourth reactor stage 407, the reactant stream ispartially converted therein, and the stream reaches equilibrium at theupper operating temperature limit of 900° C. at the outlet of stage 407.

Selected operating conditions and gas compositions are given in Table 9.

TABLE 9 Operating Conditions for Example 7 Interstage InterstageInterstage Pre-reformed feed to feed to feed to feed stage 403 stage 405stage 407 (line 430) (line 402) (line 404) (line 406) Pressure (bara) 3028 28 27 Temperature (° C.) 800 849 856 862 H₂ (mol %) 8.0 43.1 39.436.6 CO (mol %) 0.1 13.1 14.9 16.0 CO₂ (mol %) 3.2 10.6 11.9 13.0 H₂O(mol %) 52.6 27.9 29.3 30.3 CH₄ (mol %) 35.7 4.7 3.7 3.0 N₂ (mol %) 0.40.6 0.9 1.1

Conditions at the inlet to first stage 401 are similar to those inExample 5 (above). In the interstage reactant gas in line 402, theequilibrium partial pressure of oxygen is 2.7×10⁻¹⁸ bara, p_(CO2)* is3.7 bara, and the actual equilibrium value of p_(CO2) is only 2.6 bara.In the interstage reactant gas in line 404, the equilibrium partialpressure of oxygen is 4.3×10⁻¹⁸ bara, P_(CO2)* is 4.3 bara, and theactual equilibrium value of P_(CO2) is only 3.0 bara. Finally, in theinterstage reactant gas in line 406, the equilibrium partial pressure ofoxygen is 7.9×10⁻¹⁸ bara, p_(CO2)* is 5.2 bara, and the actualequilibrium value of p_(CO2) is only 3.4 bara.

It is seen from these results that p_(CO2) remains below p_(CO2)* at alllocations in the reactor system. Thus, multiple stages, in this examplefour stages with three interstage injection points, enable the injectionof this particular CO₂-containing pre-reformed F-T offgas whilemaintaining p_(CO2)<p_(CO2)* throughout the ITM reactor system.

Example 8

For membranes with compositions having less robust p_(CO2)* behaviorthan shown in FIG. 5, the stream of pre-reformed F-T offgas can bedivided and injected at a greater number of interstage injection pointsto mitigate temperature depression and corresponding decrease inp_(CO2)* at each injection point. Alternatively, the pre-reformed F-Toffgas streams can be heated before injection, for example by heaters447, 449, and 451 of FIG. 4 or in a common heater (not shown) downstreamof pre-reformer 437. The number of stages and the heating of theintermediate pre-reformed F-T offgas streams may be selected to optimizethe membrane reactor operation and equipment cost for specific feed gascompositions and product synthesis gas H₂/CO ratio requirements.

As an example, if the F-T off-gas recycle stream in line 441 of FIG. 4in Example 6 were heated from 560° C. to 650° C. in heater 447, thecombined stream in line 402 would be at 847° C. At this temperature, theequilibrium partial pressure of oxygen would be 2.6×10⁻¹⁸ bara, p_(CO2)*would be 3.7 bara, and the actual equilibrium value of p_(CO2) would beonly 2.9 bara. If the F-T off-gas recycle stream in line 443 of Example6 were heated from 560° C. to 650° C. in heater 449, the combined streamin line 404 would be at 857° C. At this temperature, the equilibriumpartial pressure of oxygen would be 5.0×10⁻¹⁸ bara, p_(CO2)* would be4.5 bara, and the actual equilibrium value of p_(CO2) would be only 3.3bara. Thus the use of two intermediate injection points and athree-stage reactor system would maintain p_(CO2)<p_(CO2)* throughoutthe ITM reactor system. The choice of reheating versus additionalreactor stages would depend on the cost tradeoff between hightemperature heat exchanger tubing for the reheat duty and the cost ofproviding stages required for the injection and mixing points.

The invention claimed is:
 1. A method for generating an oxidationproduct gas comprising (a) providing an ion transport membrane oxidationsystem comprising (1) two or more membrane oxidation stages, each stagecomprising a reactant zone, an oxidant zone, one or more ion transportmembranes separating the reactant zone from the oxidant zone, a reactantgas inlet region, a reactant gas outlet region, an oxidant gas inletregion, an oxidant gas outlet region wherein the one or more iontransport membranes are mixed conductor membranes; (2) an interstagereactant gas flow path disposed between each pair of membrane oxidationstages, wherein the interstage reactant gas flow path is adapted toplace the reactant gas outlet region of a first stage of the pair inflow communication with the reactant gas inlet region of a second stageof the pair such that interstage reactant gas can flow from the firststage to the second stage; and (3) one or more reactant interstage feedgas lines, each line being in flow communication with any interstagereactant gas flow path or with the reactant zone of any membraneoxidation stage receiving interstage reactant gas; (b) introducing oneor more reactant feed gases into the reactant gas inlet region of afirst stage of the two or more membrane oxidation stages; (c)introducing an oxidant gas into any of the oxidant gas inlet regions ofthe two or more membrane oxidation stages; (d) introducing a reactantinterstage feed gas into any of the interstage reactant gas flow pathsdisposed between adjacent membrane oxidation stage or into any reactantzone of any stage receiving interstage reactant gas wherein the reactantinterstage feed gas has a different composition than the one or morereactant feed gases introduced into the reactant gas inlet region of thefirst stage; and (e) withdrawing an oxidation gas product from thereactant gas outlet region of a last stage of the two or more membraneoxidation stages.
 2. The method of claim 1 wherein the one or morereactant feed gases comprise a first reactant feed gas and a secondreactant feed gas, wherein the first reactant feed gas comprises methaneand a second reactant feed gas comprises steam.
 3. The method of claim 1wherein each stage of the two or more membrane oxidation stages furthercomprise one or more catalysts, and wherein each stage of the two ormore membrane oxidation stages having the one or more catalysts isoperated to control a temperature approach to equilibrium with respectto the steam reforming reaction in a range from 0° F. to 100° F.
 4. Themethod of claim 3 wherein the temperature approach to equilibrium withrespect to the steam reforming reaction is controlled in a range from 0°F. and 20° F.
 5. The method of claim 1 wherein each stage of the two ormore membrane oxidation stages further comprise one or more catalysts,and wherein each stage of the two or more membrane oxidation stageshaving the one or more catalysts is operated to control a temperatureapproach to equilibrium with respect to the steam reforming reaction tobetween 0° F. and 100° F. by controlling a hydrocarbon feed rate of thereactant interstage feed gas into each stage of the two or more membraneoxidation stages having the one or more catalysts.
 6. A method forgenerating an oxidation product gas comprising (a) providing an iontransport membrane oxidation system comprising (1) two or more membraneoxidation stages, each stage comprising a reactant zone, an oxidantzone, one or more ion transport membranes separating the reactant zonefrom the oxidant zone, a reactant gas inlet region, a reactant gasoutlet region, an oxidant gas inlet region, and an oxidant gas outletregion wherein the one or more ion transport membranes are mixedconductor membranes; (2) an interstage reactant gas flow path disposedbetween each pair of membrane oxidation stages, wherein the interstagereactant gas flow path is adapted to place the reactant gas outletregion of a first stage of the pair in flow communication with thereactant gas inlet region of a second stage of the pair such thatinterstage reactant gas can flow from the first stage to the secondstage; (3) one or more reactant interstage feed gas lines, each linebeing in flow communication with any interstage reactant gas flow pathor with the reactant zone of any membrane oxidation stage receivinginterstage reactant gas; (4) one or more reactant gas feed lines in flowcommunication with the reactant zone of a first stage of the two or moremembrane oxidation stages; (5) a reactant gas supply manifold in flowcommunication with one of the reactant gas feed lines to the first stageand in flow communication with any of the reactant interstage feed gaslines; and (6) a product withdrawal line adapted to withdraw anoxidation product from the reactant zone of the last stage of the two ormore membrane oxidation stages; (b) providing a reactant gas via thereactant gas supply manifold, combining a first portion of the reactantgas with another reactant gas, introducing the combined gas into thereactant zone of the first stage, and introducing additional portions ofthe reactant gas from the manifold as reactant interstage feed gas intoany of the one or more reactant interstage feed gas lines such that thereactant interstage feed gas has a different composition than thecombined gas introduced into the reactant zone of the first stage; (c)introducing an oxidant gas into any of the oxidant gas inlet regions ofthe two or more membrane oxidation stages; and (d) withdrawing anoxidation gas product from the reactant gas outlet region of the laststage of the two or more membrane oxidation stages.
 7. The method ofclaim 6 wherein the reactant gas provided via the reactant gas supplymanifold is pre-reformed natural gas and the oxidation gas product issynthesis gas comprising hydrogen and carbon monoxide.
 8. The method ofclaim 6 wherein the other reactant gas is steam.
 9. The method of claim6 wherein each stage of the two or more membrane oxidation stagesfurther comprise one or more catalysts, and wherein each stage of thetwo or more membrane oxidation stages having the one or more catalystsis operated to control a temperature approach to equilibrium withrespect to the steam reforming reaction in a range from 0° F. to 100° F.10. The method of claim 9 wherein the temperature approach toequilibrium with respect to the steam reforming reaction is controlledin a range from 0° F. and 20° F.
 11. The method of claim 6 wherein eachstage of the two or more membrane oxidation stages further comprise oneor more catalysts, and wherein each stage of the two or more membraneoxidation stages having the one or more catalysts is operated to controla temperature approach to equilibrium with respect to the steamreforming reaction to between 0° F. and 100° F. by controlling ahydrocarbon feed rate of the reactant interstage feed gas into eachstage of the two or more membrane oxidation stages having the one ormore catalysts.
 12. A method for generating an oxidation product gascomprising (a) providing an ion transport membrane oxidation systemcomprising (1) two or more membrane oxidation stages, each stagecomprising a reactant zone, an oxidant zone, one or more ion transportmembranes separating the reactant zone from the oxidant zone, a reactantgas inlet region, a reactant gas outlet region, an oxidant gas inletregion, and an oxidant gas outlet region wherein the one or more iontransport membranes are mixed conductor membranes; (2) an interstagereactant gas flow path disposed between each pair of membrane oxidationstages, wherein the interstage reactant gas flow path is adapted toplace the reactant gas outlet region of a first stage of the pair inflow communication with the reactant gas inlet region of a second stageof the pair such that interstage reactant gas can flow from the firststage to the second stage; (3) one or more reactant interstage feed gaslines, each line being in flow communication with any interstagereactant gas flow path or with the reactant zone of any membraneoxidation stage receiving interstage reactant gas; (4) one or morereactant gas feed lines in flow communication with the reactant zone ofa first stage of the two or more membrane oxidation stages; (5) areactant interstage feed gas supply manifold in flow communication withany of the reactant interstage feed gas lines; and (6) a productwithdrawal line adapted to withdraw an oxidation product from thereactant zone of the last stage of the two or more membrane oxidationstages; (b) introducing a reactant feed gas into the reactant zone ofthe first stage of the two or more membrane oxidation stages; (c)providing reactant interstage feed gas via the reactant interstage feedgas supply manifold into any of the one or more reactant interstage feedgas lines wherein the reactant interstage feed gas has a differentcomposition than the reactant feed gas; (d) introducing an oxidant gasinto any of the oxidant gas inlet regions of the two or more membraneoxidation stages; and (e) withdrawing an oxidation gas product from thereactant gas outlet region of the last stage of the two or more membraneoxidation stages.
 13. The method of claim 12 wherein the reactant feedgas and the reactant interstage feed gas comprise pre-reformed naturalgas and the oxidation gas product is synthesis gas comprising hydrogenand carbon monoxide.
 14. The method of claim 13 comprising introducingsteam into one of the reactant gas feed lines in flow communication withthe reactant zone of the first stage of the two or more membraneoxidation stages.
 15. The method of claim 12 wherein the reactantinterstage feed gas comprises methane and/or carbon dioxide.
 16. Themethod of claim 15 wherein the oxidation gas product is synthesis gascomprising hydrogen and carbon monoxide.
 17. The method of claim 16wherein the synthesis gas comprising hydrogen and carbon monoxide isutilized as feed gas to a hydrocarbon synthesis process or an oxygenatedhydrocarbon synthesis process that generates a process offgas comprisingcarbon dioxide, and wherein some or all of the process offgas providesat least a portion of the reactant interstage feed gas.
 18. The methodof claim 16 wherein the reactant interstage feed gas is provided bypre-reforming a reactant gas comprising carbon dioxide, methane, and oneor more hydrocarbons heavier than methane, and wherein the synthesis gascomprising hydrogen and carbon monoxide is utilized as feed gas to ahydrocarbon synthesis process or an oxygenated hydrocarbon synthesisprocess that generates a process offgas comprising carbon dioxide, andwherein some or all of the process offgas provides at least a portion ofthe reactant gas that is pre-reformed to provide the reactant interstagefeed gas.
 19. The method of claim 12 wherein each stage of the two ormore membrane oxidation stages further comprise one or more catalysts,and wherein each stage of the two or more membrane oxidation stageshaving the one or more catalysts is operated to control a temperatureapproach to equilibrium with respect to the steam reforming reaction ina range from 0° F. to 100° F.
 20. The method of claim 19 wherein thetemperature approach to equilibrium with respect to the steam reformingreaction is controlled in a range from 0° F. and 20° F.
 21. The methodof claim 12 wherein each stage of the two or more membrane oxidationstages further comprise one or more catalysts, and wherein each stage ofthe two or more membrane oxidation stages having the one or morecatalysts is operated to control a temperature approach to equilibriumwith respect to the steam reforming reaction to between 0° F. and 100°F. by controlling a hydrocarbon feed rate of the reactant interstagefeed gas into each stage of the two or more membrane oxidation stageshaving the one or more catalysts.
 22. A method for generating anoxidation product gas comprising (a) providing an ion transport membraneoxidation system comprising (1) two or more membrane oxidation stages,each stage comprising a reactant zone, an oxidant zone, one or more iontransport membranes separating the reactant zone from the oxidant zone,a reactant gas inlet region, a reactant gas outlet region, an oxidantgas inlet region, and an oxidant gas outlet region wherein the one ormore ion transport membranes are mixed conductor membranes; (2) aninterstage reactant gas flow path disposed between each pair of membraneoxidation stages, wherein the interstage reactant gas flow path isadapted to place the reactant gas outlet region of a first stage of thepair in flow communication with the reactant gas inlet region of asecond stage of the pair such that interstage reactant gas can flow fromthe first stage to the second stage; (3) one or more reactant interstagefeed gas lines, each line being in flow communication with anyinterstage reactant gas flow path or with the reactant zone of anymembrane oxidation stage receiving interstage reactant gas; (4) one ormore reactant gas feed lines in flow communication with the reactantzone of a first stage of the two or more membrane oxidation stages; (5)a reactant interstage feed gas supply manifold in flow communicationwith any of the reactant interstage feed gas lines; and (6) a productwithdrawal line adapted to withdraw an oxidation product from thereactant zone of the last stage of the two or more membrane oxidationstages; (b) introducing a reactant feed gas into the reactant zone ofthe first stage of the two or more membrane oxidation stages; (c)providing reactant interstage feed gas via the reactant interstage feedgas supply manifold into any of the one or more reactant interstage feedgas lines wherein the reactant interstage feed gas has a differentcomposition than the reactant feed gas; (d) introducing an oxidant gasinto any of the oxidant gas inlet regions of the two or more membraneoxidation stages; and (e) withdrawing an oxidation gas product from thereactant gas outlet region of the last stage of the two or more membraneoxidation stages; wherein the reactant interstage feed gas comprisesmethane and/or carbon dioxide, and wherein the method comprisesmaintaining the partial pressure of carbon dioxide in the interstagereactant gas flowing into any membrane oxidation stage to be less than acritical threshold carbon dioxide partial pressure, p_(CO2)*, whereinp_(CO2)* is defined as a carbon dioxide partial pressure above which thematerial in the ion transport membranes reacts with carbon dioxide anddecomposes.